How to Design a Follow-Up PSLE Science Investigation When Two Explanations Still Fit

Wait, What? Sometimes the First Experiment Does Not Settle the Question

A plant grows less well in Setup B than in Setup A. One learner says the difference is caused by less light. Another says it is caused by less water.

Both explanations fit what has happened so far.

The next scientific move is not to argue harder. It is to design a test that makes the two explanations predict different outcomes.

A good follow-up investigation does not merely collect more data. It collects the kind of data that can separate competing explanations.

This is one of the quietest but most powerful forms of scientific reasoning. The first investigation tells you what happened. The follow-up investigation is designed around what you still do not know.

Quick Answer

When two explanations still fit the evidence, write down what each explanation predicts. Find a condition under which the predictions differ. Then design the smallest fair comparison that changes that discriminating condition, measures a relevant outcome, keeps competing conditions comparable and states in advance what result would support, weaken or leave each explanation unresolved.

TWO EXPLANATIONS → TWO DIFFERENT PREDICTIONS → ONE DISCRIMINATING TEST → RELEVANT MEASUREMENT → FAIR COMPARISON → RESULT → UPDATE THE EXPLANATION.

The Exact PSLE Science Learning Job This Guide Owns

This guide owns one learner job: how a Primary 5 or Primary 6 learner designs the next investigation after the current evidence is not enough to choose between two scientifically plausible explanations.

It does not replace the general guide on variables and fair tests. It does not replace the guide on choosing between competing explanations from evidence already given. It owns the next step:

What should we test next so the evidence becomes more discriminating?

Why This Fits the Current PSLE Science Frame

For examination from 2026, PSLE Science assesses the 2023 Primary Science syllabus. The official assessment objectives include applying scientific facts, concepts and principles; making predictions and formulating hypotheses; interpreting and analysing information; evaluating observations, information and methods; and communicating explanations and reasoning.

A follow-up investigation combines all of these. The learner must understand the competing explanations, predict what each would lead to, select a test that can distinguish them, judge the quality of the evidence and explain what the new result changes.

Start With the Unresolved Difference

Do not begin by inventing new apparatus. Begin with the unresolved scientific question.

Write:

We observed ______. Explanation A says this happened because ______. Explanation B says it happened because ______. The current evidence does not yet separate them because ______.

That final blank matters. If you cannot state why the first evidence is insufficient, you are not ready to design the next test.

The Prediction Split

A useful follow-up test is built where the explanations disagree.

Explanation	If it is correct, what should happen?
A	Prediction A
B	Prediction B

If both explanations predict exactly the same result under your proposed test, the test is not discriminating. It may produce data, but it will not answer the unresolved question.

Worked Example 1 — Light or Water?

Original practice scenario: Two identical young plants are observed over several days. Plant A receives bright light and regular watering. Plant B receives less light and less water. Plant B grows less.

Two explanations fit:

Explanation A: Plant B grew less because it received less light.
Explanation B: Plant B grew less because it received less water.

The original comparison changed two conditions at once. It cannot isolate either cause.

A follow-up can hold water supply comparable while changing light level. If the plants now differ in growth under the tested conditions, the result gives evidence about the light explanation. A separate fair comparison can hold light comparable while changing water supply.

Notice what makes this useful: each follow-up removes one ambiguity from the first investigation.

Worked Example 2 — Surface or Temperature?

Two wet cloths dry at different speeds. Cloth P is spread flat in a warmer place. Cloth Q is folded in a cooler place.

Possible explanations include exposed wet surface and temperature.

A poor follow-up is to repeat the same two cloths exactly. That may show the result is repeatable, but it still changes both factors together.

A more discriminating follow-up keeps temperature comparable and changes only how much wet surface is exposed. Another follow-up keeps exposed area comparable and changes temperature.

Repeatability and discrimination are different scientific jobs.

Worked Example 3 — Material or Thickness?

Two covers reduce heat loss differently. One is metal and thin. The other is foam and thick.

If you want to test whether material type matters, thickness must not quietly remain different. Use covers that differ in material while keeping relevant dimensions as comparable as the investigation requires.

If you want to test thickness instead, use the same material at different thicknesses.

The follow-up question decides what must change and what must stay comparable.

Worked Example 4 — Two Explanations Make Opposite Predictions

A mystery object moves farther in Setup X than Setup Y. One explanation says the difference comes from a stronger push. Another says the surface in X offers less friction.

Suppose you can keep the launching method the same and swap the surfaces between the two tracks.

If the distance difference follows the surface, that pattern is more consistent with the surface explanation. If the distance difference stays with the launching device, that pattern is more consistent with the launch explanation.

This is a powerful experimental idea: move the suspected cause while holding other parts stable and see whether the effect moves with it.

Do Not Test the Explanation by Restating It

“Test whether more light causes more growth by giving Plant A more light and Plant B less light” is incomplete if water, pot size, plant type and starting condition also differ.

The investigation must be built around an interpretable comparison, not merely around the words in the explanation.

The Five-Part Follow-Up Design

Question: What uncertainty are you trying to remove?
Prediction split: What different result does each explanation predict?
Changed condition: Which factor must differ to test that prediction?
Measured outcome: What observation or measurement would reveal the effect?
Controls: Which other conditions could otherwise produce a competing explanation?

What Makes a Follow-Up Test Discriminating?

A discriminating test has a useful asymmetry: different explanations do not all survive every possible result equally well.

Before running the test, ask:

If Result 1 appears, which explanation becomes stronger?
If Result 2 appears, which explanation becomes weaker?
Is there a possible result that leaves both explanations open?

If every result leaves both explanations equally plausible, redesign the test.

Do Not Pretend One Follow-Up Proves Everything

A well-designed follow-up can make one explanation better supported than another. It may not establish a universal law.

Your conclusion is still limited by:

the tested range;
the quality of the measurements;
the organisms or materials used;
uncontrolled conditions;
sample size and repeats;
whether the chosen outcome really reflects the process of interest.

Negative Results Can Still Be Useful

Suppose your follow-up changes light level while keeping water comparable, but both plants grow similarly within the observation period.

That does not automatically prove light has no effect on plants. It tells you that this test did not show the predicted difference under these conditions and over this duration.

The result may weaken a specific explanation of the original difference without supporting a universal “light does not matter” claim.

Choose the Smallest Useful Change

Good follow-up investigations are often simpler than the first experiment.

If the unresolved question is whether surface material matters, do not change material, slope, object mass and starting position together. Strip the design down until the evidence has one clear job.

Use the Same Measurement Language Across Set-Ups

Do not measure temperature in one setup and describe “warmness” in another. Do not measure distance travelled for one object and time taken for the other unless the question specifically requires different outcomes.

A follow-up becomes easier to interpret when the same quantity is measured consistently across compared conditions.

The Earliest-Weak-Link Diagnostic

Failure signature	Earliest weak link	Repair
“I will just repeat the first experiment.”	Repeatability was confused with discrimination.	Identify what uncertainty remains and change the design to separate the explanations.
“My follow-up changes three things.”	The learner has not isolated the prediction split.	Choose one discriminating condition and control alternatives.
“Both explanations predict the same result.”	The proposed test cannot separate them.	Find a condition where their predictions differ.
“I know which explanation is right before the test.”	Expectation has replaced evidence.	Write possible outcomes and what each would mean before observing the result.
“No difference means the factor never matters.”	A bounded result became a universal claim.	Keep the conclusion within the tested setup, range and measurement sensitivity.
“I changed the right factor but measured the wrong thing.”	The outcome does not answer the scientific question.	Choose an observation or measurement directly connected to the predicted effect.

Misconception Repair — More Data Is Not Always Better Data

Collecting ten more measurements from an ambiguous design can leave the same ambiguity. Scientific quality depends on what the evidence can distinguish, not only how much of it you have.

Misconception Repair — A Control Is Not Automatically the Follow-Up

A control or reference setup can be essential, but the follow-up question decides what reference is needed. Do not add a “control” by habit without explaining what comparison it enables.

Misconception Repair — One Explanation Can Be Weakened Without the Other Being Proven

If a result does not match Explanation A, A may become less plausible. Explanation B does not automatically become certain. A third explanation may still exist.

How This Appears in Multiple Choice

Identify what the first experiment cannot distinguish.
Read each proposed follow-up and mark what it changes.
Reject options that change several relevant conditions at once.
Check whether the measured outcome answers the unresolved question.
Prefer the option that makes competing explanations predict different results.

How This Appears in Structured Questions

A useful planning scaffold is:

To distinguish whether ______ or ______ explains the result, keep ______ comparable, change ______, measure ______, and compare ______. If ______ happens, this would be more consistent with ______ because ______.

This is not an official PSLE phrase. Use only the parts the question needs.

Practice Sequence

Take five ambiguous investigations in which two relevant conditions changed.
Write two plausible explanations for each result.
Write one prediction from each explanation.
Design a follow-up in which the predictions differ.
Identify changed, measured and controlled conditions.
Write two possible outcomes and what each would mean.
Add an unfamiliar context with the same reasoning structure.
Return several days later and design the next test without prompts.

Unfamiliar Transfer Challenge

Two identical containers lose different amounts of water. Container A is wider and is placed in moving air. Container B is narrower and is placed in still air.

Two explanations fit: exposed surface area and air movement.

Design one follow-up that tests the surface-area explanation and another that tests the air-movement explanation. For each, state what must remain comparable, what will change, what will be measured and what result would be relevant.

The transfer skill is not remembering a cloth or plant example. It is recognising an unresolved causal comparison and designing evidence that can separate it.

Delayed Independent Return

Four days later, use a fresh investigation and answer without notes:

What was observed?
Which two explanations still fit?
Why does the current evidence fail to choose between them?
What does each explanation predict?
Where do those predictions differ?
What is the smallest fair comparison that tests that difference?
What outcome should be measured?
What result would favour, weaken or leave each explanation unresolved?

The Answer-Checking Receipt

Did I identify the exact uncertainty left by the first evidence?
Did I state two plausible explanations?
Did I derive a prediction from each?
Does my follow-up make the predictions differ?
Did I change one discriminating condition?
Did I keep competing conditions comparable?
Did I choose a relevant measurement?
Did I state what different results would mean?
Did I avoid claiming universal proof from one bounded follow-up?

Useful Internal Routes

Parent and Tutor Teaching Guide

When a child offers two possible reasons, resist the urge to reveal which one you think is correct. Ask instead:

“What could we change so these two ideas stop predicting the same thing?”

Then make the learner state the result expected under each explanation before the test is discussed further. This prevents hindsight reasoning.

If the learner designs an overcomplicated investigation, ask them to remove one change at a time until the comparison becomes interpretable. If they simply repeat the original ambiguous setup, ask what uncertainty the repeat would actually remove.

Return later with a different topic. Mastery is shown when the learner spontaneously turns unresolved explanation into a discriminating next test.

Authoritative and Research References

The research references support broader scientific-reasoning and inquiry principles. They are not PSLE-specific marking rules.

The Quiet Ending

When two explanations survive, Science is not stuck.

It has simply reached the point where the next question matters more than the last answer.

Small Group Tutorials