Series ID: EDKS-ADV-VOC-ZH-0040 · Advanced English Vocabulary (Chinese Edition) · Lesson No.040 · C1 → C2 · 简体中文辅助
Count asks how many. Frequency asks how often. Prevalence asks how common. Incidence asks how often new cases occur in the defined population and time.
count 问有多少；frequency 问多常发生；prevalence 问有多普遍；incidence 问在定义人群和时间内新事件以什么频率发生。
Advanced learners often know the words frequent, common, recurrent, prevalent, incidence, yet use them as if they were different ways of saying “it happens a lot.” That collapses several distinct relationships. A condition can be common because many people currently have it, even if few new cases arise. An event can happen frequently but affect only a small group. A problem can recur many times for one system without being prevalent across the whole population. A high event count can reflect a large population rather than high occurrence rate.
CDC epidemiology materials separate these relationships clearly. Incidence concerns new cases or events arising over a specified period in a defined population; prevalence concerns cases already present at a point or during a period, including new and pre-existing cases. CDC also notes that prevalence reflects both incidence and duration: a long-lasting condition can have high prevalence even when the flow of new cases is modest.
A common condition is not necessarily a frequently recurring event.
一个很普遍的状态，不一定意味着它会频繁重复发生。
This lesson owns the Mandarin-supported C1–C2 lexical system for frequency, occurrence, recurrence, prevalence, incidence, commonness, rarity, repetition and regularity. It coordinates with Lesson No.035 on probability, Lesson No.036 on trends over time, Lesson No.037 on rate/proportion architecture, and Lesson No.039 on frequency distributions, but it does not replace them.
Health-domain caution: prevalence and incidence have specialised epidemiological meanings. This lesson teaches their language architecture and common distinctions, not medical diagnosis or personal-health interpretation.
Part I — Build the occurrence map | 第一部分：建立“发生与频率地图”
1. Occurrence names the fact or event of happening | occurrence = 发生/出现
An occurrence is an event or instance of something happening. Cambridge uses the noun for the fact of something existing or happening and for a particular event. “Three occurrences of the error were recorded” counts events. It does not yet tell us whether three is frequent because frequency requires a reference period, opportunity or comparison.
2. Occur is neutral event language | occur
Occur is more formal/neutral than happen in many professional contexts: “The failure occurred during startup.” “The event occurred twice.” It can also mean exist/be found: “This species occurs in coastal regions.” Do not assume every occurrence is temporal repetition.
3. Happen is broad everyday event language | happen
Happen is common in conversation: “How often does this happen?” In formal reports, occur may fit better. The difference is often register rather than logic. Learners should be able to move between plain and technical phrasing without changing event structure.
4. Arise means come into existence, often as a problem/question | arise
“Problems may arise.” “A question arises.” Arise focuses on emergence rather than repetition. It is not a synonym for incidence. An issue can arise once; incidence describes new-case occurrence in a defined population/time under technical use.
5. Appear can mean become visible, not necessarily begin | appear
“A rash appeared” may mean it became visible; “a pattern appeared in the data” means became evident. Appearance does not always mark the true onset of the underlying condition. This distinction matters in surveillance and diagnostics.
6. Emerge suggests becoming noticeable/developing | emerge
“A new pattern emerged.” “Several problems emerged during testing.” The verb is often used for developing evidence, issues or trends. It does not quantify frequency.
7. Event count asks how many events occurred | count
If a server failed ten times, ten is an occurrence count. Without operating hours, transaction volume or observation period, we cannot judge whether ten failures are frequent. Count is the numerator; frequency/rate requires more context.
8. Frequency asks how often | frequency
Frequency concerns how often something occurs or repeats. It may be expressed qualitatively—frequent, occasional, rare—or quantitatively—events per hour, number of occurrences in a period, relative frequency. The exact mathematical meaning depends on domain.
9. Frequency is not raw count | count vs frequency
Ten incidents in one hour is more frequent than ten incidents in one year. Same count, different frequency because the observation period changes. A sentence such as “incidents increased” should distinguish count from occurrence frequency.
10. Frequency can be relative to opportunities | per opportunity
A defect can occur ten times in 100 units or ten times in one million units. Per-unit occurrence differs dramatically. Lesson No.037 owns the denominator mechanics; Lesson No.040 asks what “frequent” means once exposure/opportunity is considered.
11. Frequent means happening often | frequent
“Frequent errors” means errors occur often, not necessarily that many distinct systems are affected. One user can experience frequent failures while the failure is rare across the entire user population.
12. Common means widespread or often encountered | common
Common often asks how widespread or usual something is across cases: “This error is common among beginners.” It can overlap with frequent, but commonness is often cross-sectional while frequency often foregrounds repetition over opportunities/time.
13. Common and frequent are not identical | common vs frequent
A migraine can be common across a population but infrequent for a particular person. A rare machine defect can recur frequently in one faulty machine. “Common” asks how broadly encountered; “frequent” asks how often it occurs in the relevant unit.
14. Widespread foregrounds geographic/social extent | widespread
Widespread says something exists across many places/people. A widespread condition may not recur frequently in each individual. It is a distribution/prevalence-like adjective rather than an event-frequency word.
15. Prevalent means common/widespread in general English | prevalent
Cambridge uses prevalent for something existing very commonly or happening often, especially over an area/time. In health/epidemiology, prevalence is more technical: the proportion/number of existing cases in a defined population at a point or period.
16. Prevalence is a state/burden measure in epidemiology | prevalence
CDC distinguishes prevalence from incidence by including new and pre-existing cases present at the relevant time. Conceptually, prevalence asks “How much of the population currently/over this period has the condition?” It is therefore closer to existing burden/commonness than to new-case flow.
17. Incidence focuses on new occurrence | incidence
CDC defines incidence around new cases/events arising during a specified period in a population at risk. The numerator contains new cases, not all existing cases. Depending the measure, incidence may be expressed as a proportion or a person-time rate.
18. New versus existing is the core incidence–prevalence contrast | 新发 vs 现存
If 100 people currently have a condition, those existing cases contribute to prevalence. If 20 of them first developed it during the period, those 20 may contribute to incidence under the defined measure. Same condition, different numerator question.
19. Point prevalence asks “present at this point” | point prevalence
Point prevalence uses cases present at a specified point in time divided by the relevant population at that point, under the formal epidemiological definition. It is a snapshot concept.
20. Period prevalence asks “present at any time during this interval” | period prevalence
Period prevalence includes people/cases present during a stated interval, including cases that began before the interval and persisted into it under standard definitions. It is not the same as new cases during the interval.
21. Duration affects prevalence | duration
CDC notes that prevalence depends on incidence and duration. A long-lasting condition can accumulate many existing cases even when relatively few new cases arise each year. Conversely, a short-duration condition can have substantial incidence but modest prevalence at any snapshot.
22. High prevalence does not prove high incidence | prevalence ≠ incidence
A chronic long-duration condition can be highly prevalent with moderate new-case incidence. Saying “many people have it, therefore many new cases are occurring” is not logically safe without incidence data.
23. High incidence does not guarantee high point prevalence | short duration
If events/conditions resolve quickly, many new cases can occur over time while relatively few exist at any single point. The reservoir may refill and drain quickly. New flow and existing stock are different concepts.
24. Recur means happen again | recur
Recur means occur again, especially after stopping or after an interval. “The error recurred after the update.” Recurrence concerns repeat episodes, not cross-sectional commonness.
25. Recurrence is repeat occurrence | recurrence
A recurrence is another occurrence of the same/similar event or condition. One system can experience recurrent failures even if only that system is affected. Recurrence belongs to event history.
26. Recurrent and recurring overlap | recurrent vs recurring
Both adjectives describe things that happen repeatedly. Recurrent often sounds formal/medical/technical: recurrent infection, recurrent problem. Recurring is common in general/professional English: recurring payment, recurring meeting, recurring issue. Collocation and domain choose between them.
27. Repeated means happened more than once | repeated
Repeated is broad and does not necessarily imply a stable recurring pattern. “Repeated errors occurred during testing” means more than one event. “Recurrent errors” can suggest a problem keeps returning.
28. Repeat can be noun/adjective/verb depending construction | repeat
“Repeat occurrence,” “repeat customer,” “repeat the test.” It is flexible but often less formal than recurrence. Avoid “repeat incidence” unless the field uses it conventionally.
29. Relapse is medical/behavioural return after improvement | relapse
Relapse is commonly used when a disease, condition or behaviour returns after improvement/recovery. It is not a general synonym for recurrence in every domain. A server failure recurs; it does not “relapse.”
30. Reappearance is neutral return-to-view language | reappear
A symptom, error message or pattern can reappear. The word focuses on visibility/presence, not necessarily whether the underlying process truly stopped and restarted.
31. Occasional means happening sometimes, not often | occasional
An occasional event occurs from time to time. It is more frequent than “never” but does not imply a formal rate. Context determines what counts as occasional.
32. Infrequent means not often | infrequent
Infrequent directly contrasts with frequent. It says recurrence/occurrence is low relative to the reference period/opportunities, but does not mean impossible or isolated.
33. Rare means uncommon, not impossible | rare
Rare describes low commonness or low occurrence frequency. A rare event remains possible. Lesson No.035’s probability boundary applies: rarity is usually empirical/commonness language, not logical impossibility.
34. Sporadic means occasional and irregular/scattered | sporadic
Sporadic often describes events occurring occasionally at irregular intervals or scattered locations. “Sporadic failures” suggests no stable continuous pattern. In epidemiology, sporadic can have more specific descriptive uses.
35. Intermittent means stopping and starting | intermittent
An intermittent signal, pain or connection repeatedly stops and starts. It describes temporal pattern/duration, not population commonness. A problem can be intermittent but frequent during each day.
36. Continuous is the contrast to intermittent | continuous
Continuous means without interruption over the relevant interval. Continuous presence can create high prevalence even without frequent recurrence because the state does not end between episodes.
37. Persistent means continuing/recurring over time | persistent
Persistent can describe a state that continues or a problem that keeps recurring. It foregrounds durability. A persistent condition is not automatically frequent if it is one continuous episode.
38. Chronic is domain-sensitive duration language | chronic
In health contexts, chronic has medical meanings concerning duration/course. In general language it can mean long-lasting or persistent. Do not use it as a casual synonym for frequent.
39. Acute is not “high frequency” | acute
Acute commonly describes sudden/severe/short-course health conditions or intense problems. It is not the opposite of rare/frequent. Duration/severity and frequency are separate dimensions.
40. Endemic, epidemic and pandemic are specialist epidemiological terms | domain caution
These terms describe disease occurrence patterns relative to populations/regions and expected levels under epidemiological definitions. They should not be used casually as generic synonyms for common, widespread or frequent without respecting public-health meaning.
41. Regular means occurring according to a consistent pattern | regular
A weekly event can be regular but not frequent relative to a daily event. Regularity describes predictability/pattern; frequency describes how often. “Regular but infrequent” is coherent.
42. Irregular means timing/pattern is inconsistent | irregular
An irregular event can still be frequent if it happens many times unpredictably. Do not equate irregular with rare.
43. Periodic means repeating at intervals | periodic
Periodic implies repeated occurrence separated by intervals, often with some regularity. In mathematics/physics it can have formal meanings. In general English, periodic review may occur monthly/annually.
44. Seasonal ties recurrence to calendar seasons/periods | seasonal
A seasonal occurrence repeats with time of year or recurring calendar pattern. Seasonal frequency is not a long-term trend by itself. Lesson No.036 owns trend versus seasonality.
45. Part I checkpoint: separate five occurrence questions | 第一部分检查点
Ask five questions separately: How many events/cases? How often do events happen? How broadly/common is the state? How many are new? Does the same event/condition return? Those questions map roughly to count, frequency, prevalence/commonness, incidence and recurrence. If you collapse them, quantitative meaning is lost.
Part II — Frequency patterns, duration and recurrence | 第二部分：频率模式、持续时间与重复发生
46. Frequency needs a reference interval or opportunity | 频率需要时间或机会基准
“Five events” is a count. “Five events per hour” is a frequency/rate-like statement. “Five events per 1,000 transactions” normalises by opportunity. A frequency claim becomes meaningful only after the reader knows the observation window or exposure base.
47. Events per hour and events per user answer different questions | denominator family
Ten failures per hour measures temporal occurrence. Ten failures per 1,000 users measures population exposure. Ten failures per 10,000 transactions measures opportunity. All can describe “failure frequency,” but their denominators serve different decisions.
48. “Daily” can mean once per day or occurring on many days | daily
Daily may describe a schedule (“a daily backup”) or an observation period (“daily error count”). It does not automatically mean exactly one occurrence every day. “The symptom occurred daily” generally means on each/most days, while “once daily” specifies one occurrence/dose per day.
49. Once, twice, three times quantify recurrence directly | explicit count
When exact recurrence count is known, explicit language is often better than vague “frequent”: “The failure occurred four times in two hours.” Frequency adjectives should not replace available measurements.
50. Repeatedly means more than once, often many times | repeatedly
“The system repeatedly failed” foregrounds recurrence but not exact count or schedule. It may sound more emphatic than “failed several times.” Use it when repetition itself matters, not to imply a formally measured high frequency.
51. Frequently means often, not continuously | frequently vs continuously
A frequent interruption happens often but has gaps. A continuous interruption makes little semantic sense because interruption itself implies discontinuity. More importantly, frequent symptoms can be episodic; continuous symptoms persist without interruption.
52. Persistent state and recurrent event are different | state vs episode
A persistent error may remain present for hours. A recurrent error may disappear and return ten times. Both can create substantial burden, but one is long duration and the other repeated episodes. This distinction is critical for prevalence reasoning.
53. Episode is a bounded occurrence | episode
An episode is a distinct period/event: an outage episode, episode of pain, episode of abnormal behaviour. Recurrence requires defining when one episode ends and another begins. Without that rule, counting recurrences can be arbitrary.
54. Duration asks how long one episode lasts | duration
Two systems can have the same event frequency but very different total downtime if episodes differ in duration. “Outages occurred twice weekly” does not tell us whether each lasted one second or six hours.
55. Frequency × duration shapes time burden | burden architecture
Many short episodes and a few long episodes can produce similar total affected time. Frequency and duration are separate axes. This logic helps explain why prevalence can remain high when duration is long even if new-case incidence is modest.
56. Intermittent describes on-and-off presence | intermittent
An intermittent connection alternates between working and failing. The term describes pattern of presence and absence. It does not specify whether interruptions occur regularly or how many times per hour.
57. Episodic describes occurrence in distinct episodes | episodic
Episodic is useful when events come in bounded episodes rather than continuously. It is common in medical and behavioural contexts and may appear in technical incident descriptions. Do not use it as a synonym for rare; episodes can be frequent.
58. Periodic means repeating at intervals | periodic
Periodic events recur at intervals, often with some regularity. A weekly maintenance cycle is periodic. In science and mathematics, periodic can have exact meanings involving a period; general English can be looser.
59. Interval is time/space between occurrences | interval
“At two-hour intervals” specifies recurrence spacing. “Intervals varied from two to six hours” describes irregular spacing. Frequency can be inferred from interval, but interval and frequency are inverse-style concepts rather than synonyms.
60. Cycle describes a repeated sequence of phases | cycle
A cycle contains stages that recur, not merely repeated identical events. Lesson No.036 owns cyclical trend language; Lesson No.040 treats cycle as recurrence architecture: the system returns through a sequence.
61. Seasonal recurrence is tied to calendar/environment | seasonal
An event can be frequent during one season and absent in another. Annual prevalence/incidence/frequency summaries may hide seasonality. Specify the seasonal window when occurrence patterns matter.
62. Clustered events occur close together | temporal clustering
Ten events in ten consecutive minutes and ten events evenly spread across a year have the same count but different temporal clustering. “Clustered occurrence” describes bunching in time. Lesson No.039 owns cross-sectional clusters; here the cluster is temporal recurrence.
63. Burst describes short period of intense activity | burst
A burst of errors/requests/events means a concentrated short episode of high frequency. It is vivid professional/technical language. “Bursty traffic” is common in computing but more specialised.
64. Sporadic means irregularly scattered occurrence | sporadic
Sporadic events occur occasionally without a regular pattern. They may be widely separated in time or place. “Sporadic outages” contrasts with scheduled periodic outages and persistent instability.
65. Isolated means separate, not necessarily rare overall | isolated
An isolated incident suggests a single/separate event not part of a broader pattern. One isolated event in this system can coexist with high event frequency elsewhere. The scope matters.
66. One-off means not expected to recur | one-off
One-off is common British/international English for something happening once or created for one occasion. “A one-off error” implies non-recurring status, which should be evidence-based rather than assumed after one observation.
67. Recurrent means pattern of return, not just two events | recurrent
Two occurrences technically show repetition, but recurrent often suggests an established pattern of return. Use repeated for minimal factual recurrence when pattern strength is uncertain.
68. Recurrence interval measures time between episodes | recurrence interval
In engineering, medicine and natural hazards, recurrence intervals may be defined formally. General English can simply say “time between episodes.” Do not assume the term means perfectly regular periodicity.
69. Return period is technical in hazard fields | return period
Hydrology and risk fields use return period technically. A “100-year event” does not mean it occurs exactly once every 100 years. Such terminology requires domain interpretation rather than ordinary calendar intuition.
70. Frequency and probability are different | frequency vs probability
Frequency is observed/repeated occurrence over opportunities/time; probability is a model/judgment of chance for an event. Historical frequency may inform probability estimates, but they are not identical. Lesson No.035 owns probability language.
71. Rare event can have high probability under new conditions | history vs current risk
An event rare historically may become likely if conditions change. “Rare” describes empirical/commonness history or distribution; “unlikely” describes current probability judgment. Do not convert one automatically into the other.
72. Frequent event can be low severity | frequency vs severity
Minor warnings may occur hourly and cause little harm. A catastrophic failure may occur once in years. Frequency and consequence belong to different axes. Lesson No.035 owns risk architecture.
73. Rare event can be high risk | low frequency, high impact
Low historical frequency does not justify calling risk low if consequences are severe. The correct sentence may be: “The event is rare but potentially catastrophic.”
74. Frequency and prevalence can diverge | frequency vs prevalence
A state can be prevalent because it persists, even if individual onset events are infrequent. Conversely, a short event can happen frequently while few people are affected at any given instant. Existing burden and event repetition are different.
75. Frequency and incidence can diverge across individuals | recurrent episodes vs new cases
One person can experience ten recurrent episodes. That creates high episode frequency but only one person may count as an incident case under a disease-onset definition. Incidence of people and frequency of episodes are not interchangeable numerators.
76. Person-level incidence vs event-level incidence | unit of analysis
Some studies count first events per person; others count recurrent events. The metric definition must state the unit. “Incidence of events” and “incidence of persons experiencing first event” can differ substantially.
77. New case requires onset rule | onset
To classify a case as incident/new, the study must define when onset begins. Reappearance after remission may count as recurrence rather than new person-level incidence. Domain protocols determine the rule.
78. First occurrence and recurrence should be separated | first event vs repeat
A system reliability report may count first failure and all failures differently. Health studies may distinguish first diagnosis from relapse. Use explicit labels when event history matters.
79. Recurrence rate is domain-defined | recurrence rate
“Recurrence rate” can mean the share of cases that recur within a period or a recurrent-event rate per time, depending domain. Do not infer the denominator from the words alone. Lesson No.037’s metric-card rule applies.
80. Episode frequency should state unit and horizon | episode frequency
“Three episodes per month” is clearer than “high recurrence.” If severity/duration also matter, report them separately. A complete event pattern often needs count + interval + duration.
81. Habitual means characteristic repeated behaviour | habitual
Habitual describes behaviour done regularly as a habit. It is not a statistical frequency category. “Habitual checking” may be frequent, but the adjective also implies behavioural pattern/automaticity.
82. Routine means regularly performed by design | routine
A routine check happens according to standard practice. It can be weekly or yearly. Routine status concerns procedural regularity, not high frequency.
83. Routine occurrence vs common occurrence | procedural vs empirical
A routine maintenance shutdown is expected by design; a common fault is empirically widespread. Both may happen often, but the meaning source differs.
84. Normal frequency is reference-dependent | normal frequency
“Normal frequency” can mean within expected range for a person/system. It is not the population mean automatically. Define the benchmark or reference range.
85. Baseline frequency gives a comparison reference | baseline frequency
Before an intervention, average weekly occurrence may establish a baseline. Later frequency changes should use the same event definition and observation exposure to be comparable.
86. Frequency can rise because observation increases | surveillance intensity
More monitoring can detect more events without true event occurrence increasing. Reported frequency and underlying frequency may diverge. Surveillance/testing intensity is a measurement mechanism, not necessarily a causal increase.
87. Frequency can fall because opportunities fall | exposure denominator
If a machine operates half as many hours, raw failures can fall even if failures per operating hour rise. Count and rate/frequency per exposure must be distinguished.
88. Reported occurrence can depend on reporting rules | reporting bias
An event may become more frequently reported because classification improves or reporting becomes mandatory. “Incidence increased” is stronger than “reported incidence/count increased” when surveillance changed.
89. Detection frequency and true frequency are different | detection
A more sensitive detector can record more occurrences without the underlying process changing. Name detected events when measurement capability matters.
90. Part II checkpoint: frequency pattern needs count, denominator, interval and duration | 第二部分检查点
A complete event-frequency description often needs four pieces: how many events, per what time/opportunity, how regularly they occur, and how long each episode lasts. Add severity separately. This prevents “frequent” from becoming a vague substitute for event structure.
Part III — Incidence and prevalence: new flow versus existing burden | 第三部分：incidence 与 prevalence——新发流量与现有负担
91. Prevalence is a stock-like concept | prevalence = 现有病例/状态负担
Conceptually, prevalence resembles a stock: how many people/cases currently have or had the condition during a defined period, relative to the relevant population. It answers a burden/commonness question. It does not tell us when each case began.
92. Incidence is a flow-like concept | incidence = 新发流量
Incidence resembles a flow: new cases/events entering the case pool during a defined period or amount of person-time. The event is onset/new occurrence. It answers how quickly new cases arise, not how many total cases exist.
93. Stock and flow can move differently | stock vs flow
A bathtub can stay full even when inflow slows if outflow is also slow. Likewise, prevalence can remain high when incidence falls if cases last a long time. This metaphor helps learners understand why prevalence and incidence need separate nouns.
94. Recovery/removal affects prevalence | exits from stock
If cases recover quickly, die, migrate, or otherwise leave the case pool under the study definition, prevalence can fall even if incidence remains stable. Prevalence depends on entry and exit dynamics.
95. Long duration can raise prevalence | chronic duration
A condition with modest annual incidence can become common if people remain cases for years. This is why “high prevalence” should not be paraphrased “many new cases each year.”
96. Short duration can keep prevalence low | short-lived conditions
A short-lived condition can generate many new episodes over a year but have low point prevalence because cases resolve quickly. High incidence and low snapshot prevalence can coexist.
97. Population at risk belongs to incidence denominator | at-risk population
Incidence measures typically use a population at risk of becoming a new case. People who already have the condition at the beginning may not be eligible to become incident cases under a first-onset definition. Technical studies define eligibility carefully.
98. Incidence proportion is a cumulative-risk style measure | incidence proportion
CDC epidemiology distinguishes incidence proportion (risk) from incidence rate. Incidence proportion relates new cases during a period to a population initially at risk under defined assumptions. It is a proportion over a stated horizon, not a person-time rate.
99. Incidence rate uses person-time | incidence rate
Incidence rate (incidence density) uses person-time or another exposure-time denominator rather than simply initial population count. This accommodates people observed for different lengths of time. Technical calculation belongs to epidemiology; the vocabulary lesson is that rate and proportion are different incidence families.
100. Person-time combines people and observation time | person-time
One person observed for ten years and ten people observed for one year each both contribute ten person-years, under a simple conceptual example. Person-time is an exposure denominator. It should never be paraphrased as “ten people.”
101. Cases per person-year are not percentages | rate scale
“5 cases per 1,000 person-years” is an incidence rate. It is not 0.5% probability without further assumptions/model conversion. Lesson No.037’s rate/proportion boundary applies strongly here.
102. Cumulative incidence needs a fixed horizon | cumulative incidence
A cumulative incidence/risk statement is incomplete without a period: 1-year risk, 5-year risk, lifetime risk. Probability increases with observation horizon under many processes, so the time window is part of meaning.
103. Lifetime prevalence differs from point prevalence | lifetime prevalence
Some fields report whether a person has ever met criteria during their lifetime. This measure is much broader than current/point prevalence. The word “prevalence” therefore needs its time frame.
104. Annual prevalence differs from annual incidence | same period, different case status
Both may use a one-year period, but prevalence includes existing/ongoing cases within the period, while incidence focuses on new onsets during that period. Same time window does not make the measures equivalent.
105. Prevalence proportion is often expressed as percentage | prevalence format
“Prevalence was 12%” usually expresses a part-to-whole proportion. Lesson No.037 reminds us that percentage is numerical format, prevalence is the epidemiological metric.
106. Prevalence count and prevalence proportion differ | count vs proportion
“10,000 people are living with X” is a prevalent-case count. “Prevalence is 2%” normalises by population. Population growth can make count rise while prevalence proportion stays stable or falls.
107. Incident case count and incidence rate differ | new-case count vs rate
“500 new cases occurred” is a count. “Incidence was 5 per 100,000 person-years” is rate-like. The first may rise because population/exposure increases even if the rate stays constant.
108. New cases can rise while incidence rate falls | population growth
If the population grows rapidly, more new cases can occur while new cases per person fall. Public communication should avoid saying “incidence rose” when only incident-case count rose.
109. Prevalence can rise because survival improves | longer duration
If people live longer with a condition, prevalence can increase even if incidence stays unchanged or falls. A rising prevalence is not automatically evidence that prevention failed; duration/survival changes can contribute.
110. Prevalence can fall because duration shortens | faster recovery
If cases resolve faster, fewer exist at any point. Prevalence may fall even while new-case flow remains similar. Again, stock and flow differ.
111. Migration changes prevalence denominator/numerator | population movement
People entering/leaving a region can change observed prevalence independent of new local incidence. Epidemiological interpretation considers population dynamics.
112. Detection and diagnosis change observed incidence | ascertainment
Better screening can increase detected new cases even if underlying onset frequency is stable. Use diagnosed/reported incidence or explain surveillance changes when they materially affect interpretation.
113. Case definition changes can break comparability | case definition
If diagnostic criteria expand, prevalence/incidence may appear to rise because more people qualify as cases. A time trend across a definition change needs qualification.
114. Screening frequency affects observed new diagnoses | detection opportunity
A heavily screened population can have higher recorded incidence because cases are found earlier/more often. Reported incidence is partly a surveillance-system output.
115. Recurrence is not incidence of first disease onset | recurrence boundary
A person who relapses may contribute a recurrence event but not another first-onset incident case under many study definitions. Studies of recurrent-event incidence use different methods and should name them.
116. Relapse rate is not prevalence | relapse
Relapse rate concerns return among people previously improved/remitted, usually within a period. Prevalence concerns how many currently/periodically have the condition in a population. Different denominator and event history.
117. Remission is exit-state vocabulary | remission
In medical contexts, remission describes reduction/disappearance of signs/symptoms under domain definitions. Recurrence after remission is a return event. General learners should recognise the sequence without using it for unrelated technical systems.
118. Endemic refers expected/background presence in a region | endemic
Public-health usage of endemic concerns a disease/agent being constantly present or at usual prevalence in a geographic population under epidemiological concepts. It is not simply “very common.”
119. Epidemic refers occurrence above expected level in a population/area | epidemic
In epidemiology, epidemic occurrence is relative to expected background in a community/region and period. A disease can be rare in absolute terms but epidemic if observed cases exceed expectation.
120. Outbreak is often localised epidemic occurrence | outbreak
Outbreak often describes a localised increase/cluster of cases. It can be used for foodborne disease or other events. Exact public-health usage should follow health authority definitions.
121. Pandemic concerns widespread epidemic across countries/continents | pandemic
Pandemic is a specialist public-health term for widespread epidemic occurrence across countries/continents or globally under accepted definitions. It does not simply mean “severe disease.” Geographic spread and transmission context matter.
122. Severity and incidence are separate | severity
A mild illness can have high incidence. A severe disease can have low incidence. “Severe outbreak” combines outcome severity and occurrence pattern; neither dimension substitutes for the other.
123. Fatality and prevalence are separate | outcome vs burden
Case fatality, mortality rate, prevalence and incidence are different health metrics with different denominators/time structures. Lesson 040 does not collapse them into “how bad/common the disease is.”
124. Mortality rate is an occurrence-rate concept | mortality
Mortality rate counts deaths relative to population/person-time under defined measures. It is not the same as case fatality proportion, which uses cases as denominator. Technical health reporting must preserve the metric name.
125. Case fatality proportion is not incidence | case fatality
Case fatality concerns the proportion of diagnosed cases that die under a period/definition. Incidence concerns new disease occurrence. A highly fatal disease can be rare; a common disease can have low fatality.
126. Attack rate is a specialised outbreak measure | attack rate
CDC uses attack rate as a variant of incidence proportion in outbreak settings. Despite the word “rate,” it is proportion-like. This is a perfect example of Lesson 037’s rule: conventional metric names can conflict with pure mathematical labels.
127. Secondary attack rate is even more specialised | secondary attack rate
This measure concerns spread among contacts of primary cases under a formal epidemiological definition. It belongs in recognition vocabulary, not general-purpose “frequency” language.
128. Surveillance prevalence is not diagnosis prevalence automatically | surveillance definitions
Different systems may define cases using symptoms, lab confirmation, administrative records or survey responses. Reported prevalence depends on case ascertainment and definition.
129. Self-reported prevalence and measured prevalence can differ | measurement method
A questionnaire asking “Have you ever been told you have X?” measures something different from laboratory-confirmed current condition. The noun prevalence needs a method description where interpretation depends on ascertainment.
130. Crude vs adjusted prevalence/incidence | adjustment
Health statistics may adjust rates/proportions for age or other population structure. “Adjusted incidence” should not be paraphrased as raw observed frequency. Method labels survive translation and summary.
131. Age-standardised rate is specialised | standardisation
Age-standardised incidence/mortality rates allow comparisons across populations with different age structures under a chosen standard. They are derived comparative measures, not actual observed individual risk.
132. Prevalence ratio and prevalence difference are comparative measures | comparison
Technical studies compare prevalence across groups using ratios/differences. These measures are not prevalence itself. Preserve the comparative noun and method.
133. Incidence rate ratio compares rates | incidence rate ratio
An incidence rate ratio divides two incidence rates. A value above 1 may indicate higher rate in one group under the comparison definition. It is not “the incidence is X% higher” without conversion/context.
134. Relative risk compares incidence proportions | risk ratio
Risk ratio is a technical epidemiological comparison. Lesson No.035 covers risk generally; Lesson No.040 simply warns that relative risk, odds ratio, incidence rate ratio and prevalence ratio are not interchangeable labels.
135. Part III checkpoint: preserve case status, denominator and time | 第三部分检查点
For health-domain occurrence language, identify new versus existing cases, population at risk, time horizon/person-time, duration, case definition, and whether recurrence is counted separately. Incidence and prevalence are technical structures, not elevated synonyms for frequent/common.
