Lesson focus: evaluationJudging how good your method and results were, and how they could be improved. asks whether the evidenceObservations or measurements used to show whether an idea is right or wrong. is strong enough to support the conclusion.
Evaluate a method and conclusion using limitationA weakness in an investigation that makes you less sure about the conclusion. and realistic improvements.
Build on earlier learning
Analysing dataRevisit how patterns, variation and anomalies are identified during analysis.
You will be successful when you can…
- Judge whether a conclusion matches the data.
- Distinguish random variation from systematic bias.
- Link each limitation to its likely effect.
- Suggest a realistic improvement targeted at the limitation.
Words that carry the science
- evaluation
- A reasoned judgement about evidence, method and conclusion.
- random error
- Unpredictable variation causing repeated values to scatter.
- systematic error
- A consistent bias shifting measurements in the same direction.
- limitation
- A feature restricting confidence or the scope of a conclusion.
- reliability
- The consistency of evidence when measurements or investigations are repeated.
Evaluate the strength and limits of evidence
01Judge claim against evidence
A conclusion should answer the question and match the direction, size and uncertainty in the results. Avoid “proved” when the evidence only supports a pattern under tested conditions.
Check range, repeats, anomalies and whether a confounder offers another explanation. These determine how widely the claim can apply.
Living things vary, so testing one plant or one person is weak evidence. That one might be unusual. Use several and compare their mean.
02Diagnose the error type
random errorUnpredictable differences between repeat measurements, which make results spread out. produces scatter, such as variable human reaction time. Repeats and a mean can reveal and reduce its influence on the summary.
systematic errorA mistake that shifts every measurement the same way, such as a scale that reads 2 g too high. shifts readings consistently, such as a balance that reads +2 g when empty. More repeats reproduce the bias; calibration or method change is needed.
03Target the improvement
Write limitation → effect → improvement. “Heat escaped, so measured cooling included uncontrolled energy transfer; add a lid and insulation” is stronger than “use better equipment”.
Improvements must be feasible and likely to matter. Adding repeats does not fix every limitationA weakness in an investigation that makes you less sure about the conclusion..
A strong evaluation makes every reasoning link explicit.
- EvidenceWhat pattern, variation or anomaly is present?
- LimitationWhat feature of method or sample restricts confidence?
- EffectHow could it change measurements or interpretation?
- ImprovementWhat targeted change would reduce that effect?
- JudgementHow confident and how broad can the conclusion be?
Repeat readings and the mean
A ruler is dropped and caught; the distance it falls gives the reaction time. Each reading has random error, so repeat it. The mean of many readings settles down, and an anomaly stands out.
Controls
Readings are illustrative; the true value (0.25 s) is never known in a real experiment. Repeats do not remove random error, but the mean of many readings is closer to the true value.
Target an improvement
A student times a 2-second event by hand once at each condition.
- Limitation: manual start and stop reaction time is large relative to the event.
- Effect: measured times may vary unpredictably and obscure condition differences.
- Improvement: use automated timing or video analysis; also repeat each condition.
- Judgement: current evidence is weak because one reading cannot show variation.
How to make the reasoning visible: Explain why the improvement changes the evidence, not only what new equipment to use.
Error diagnosis
Match each observation to the likely issue and most useful response.
| Observation | Likely issue | Targeted response |
|---|---|---|
| Repeated times scatter around a similar centre | Random variation | Repeat and calculate mean; standardise technique |
| Balance reads 2.0 g when empty | Systematic zero error | Tare or calibrate before measuring |
| One value is far from close repeats | Possible anomaly | Check record and repeat that condition |
| Only two independent-variable values | Limited range | Add safe intermediate values |
The balance reads 2.0 g with nothing on it. Why won’t doing more repeats fix this?
A test used only two temperatures. Finish this chain: problem → what it means → how to improve.
One value is far from the other repeats. What should the student do?
Check your understanding
Answer from memory, then check against the suggested answer.
What does random error do to repeated results?
What does systematic error do?
What makes a good improvement?
What is a limitation?
How can you reduce the effect of random error?
A student tests plant growth using only one plant for each condition. What is the problem, and how can they improve it?
A balance gives the same reading every time, but the reading is wrong. Can results be repeatable but still wrong? Explain.
A thermometer always reads 1 °C too high. Is this a random or systematic error? How can you tell?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student tested how temperature affects the time for sugar to dissolve. They tested 20 °C and 60 °C, once each. They concluded: “Increasing temperature always decreases dissolving time.” Answer the parts to evaluate this conclusion.
Why is the word “always” a problem?
(1)Each temperature was tested only once. Why is this a problem?
(1)Suggest how to improve the range of temperatures tested.
(1)Suggest how to make the result at each temperature more trustworthy.
(1)Give your judgement. Finish the sentence: “The evidence suggests that … but it cannot show …”
(1)Total for this question: 5 marks
Command word: For evaluateA command word: weigh up the good points and bad points, then give your overall judgement., say what is good and what is weak, then finish with your judgement.