Topic 3 of 3
Evaluate evidence
Explain limitations and target improvements.
A-Level 8873, revised syllabus (2026-2027)
Evaluate evidence and explain the direction of an error
An improvement is useful when it targets a specific limitation.
| Issue | Effect on evidence | Useful response |
|---|---|---|
| Random variation | Repeated results scatter unpredictably | Repeat independent measurements; compare their spread and use a justified mean |
| Systematic bias | Results are displaced consistently, for example by an incorrect calibration | Check against a suitable standard or correct the apparatus/method; repetition alone does not remove the bias |
| Uncontrolled variable | Two possible causes change together | Hold that variable constant or design an additional comparison |
| Limited range or sample | A relationship may be supported only locally | Collect appropriate additional values; restrict the conclusion to the evidence |
Accuracy concerns closeness to a valid reference value; precision concerns the closeness of repeated measurements. A tightly grouped set can still be inaccurate. An anomalous point is a reason to investigate the method and repeat a measurement, not permission to remove inconvenient evidence silently. Record an exclusion and its reason.
| Run | Treatment and temperature | Time to the same gas volume |
|---|---|---|
| A | Without M, 25 °C | 48 s |
| B | With M, 35 °C | 24 s |
| C | Without M, 35 °C | 25 s |
Worked example
Build an argument, then choose the next comparison
A supplier claims that adding unfamiliar material M doubles the rate of a gas-forming reaction. All runs use the same reactant amounts and apparatus. The table gives one run per condition; a time difference of 1 s is within the observed timing variation.
- A and B change both temperature and the presence of M. Their 48 s versus 24 s times cannot isolate an effect of M.
- B and C use the same temperature. Their 24 s and 25 s times differ by an amount within the stated variation, so these results do not establish an improvement caused by M.
- Higher temperature is a plausible alternative explanation: at 35 °C a larger fraction of collisions can overcome the activation energy. This explains a rate change without assuming M is a catalyst.
- Recommend repeated runs with and without M at the same controlled temperature, keeping quantities, total solution volume and the measured gas-volume endpoint fixed. Compare the spread as well as the mean times.
Do not adopt M on this evidence alone. The claim is not established; that is different from proving that M has no effect. A controlled, repeated comparison can distinguish the explanations.
Worked example
Follow an error through the calculation
An acid in a burette neutralises a known amount of alkali in a flask, in a 1:1 ratio. The burette tip initially contains an air bubble that fills with acid during the titration. What happens to the calculated acid concentration?
- Some liquid leaving the graduated barrel fills the tip instead of entering the flask. The recorded titre is larger than the acid volume delivered to the flask.
- The known alkali amount still determines the acid amount needed for neutralisation.
- Calculated acid concentration = known reacting amount / recorded acid volume. A denominator that is too large gives a concentration that is too low.
The calculated acid concentration is too low in this arrangement. Fill the tip before taking the initial reading. If the unknown were instead in the flask, the calculation and error direction would need to be reconsidered.
Write evaluation as a causal chain: limitation -> affected measurement -> effect on the calculated result -> targeted improvement. "Human error" and "use better equipment" do not identify what happened. For example, gas escaping before collection lowers measured gas volume; assembling a closed collection system before starting the reaction addresses that loss.