Lesson 3 of 7
Evaluate evidence
Explain limitations and target improvements.
SEC G2 Science K223 / K225 (2027)
I can evaluate methods and suggest improvements.Syllabus
Syllabus K223 / K225, C4. Evaluate methods and suggest improvements
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.
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.
Check your understanding
A thermometer reads 2 degrees too high at every temperature. Will averaging five readings remove that error?
Show the answer
No. Averaging reduces random scatter, but a fixed offset stays in every reading.
Every reading is 2 degrees high, so their average is too. Calibrate the thermometer against a reference. A temperature change measured with the same thermometer can cancel a constant offset.