Evaluating investigations

Every CCEA practical paper finishes with a "comment on the method / suggest improvements" question worth 3-6 marks. The marks are reserved for specific, sensible, reasoned points — vague answers like "be more careful" or "do it again" score zero.

What examiners look for

1. Identify a specific weakness in the apparatus, technique or design.
2. Explain why it matters — what error does it introduce?
3. Suggest a realistic improvement that fixes the weakness.

Types of error

• Random error — small, unpredictable variation in repeats (e.g. reaction time on a stopwatch). Reduced by repeating measurements and taking a mean (after discarding anomalies).
• Systematic error — every reading is off by the same amount in the same direction. Often caused by a poorly calibrated instrument or a zero error. Repeats do not fix it; calibrate the instrument or subtract the zero offset.
• Anomalous result — a single reading that does not fit the trend. Identify, discard when calculating the mean, and explain in the evaluation.

Limitations of the method

Did the investigation actually test the stated hypothesis? Common limitations:

• Insulation — heat lost from a calorimeter to the surroundings (uses lid, lagging).
• Reaction-time error — student starts/stops a stopwatch by hand (use light gates / data logger).
• Resolution too low — measuring length with a ruler when callipers would resolve to 0.02 mm.
• Sample size too small — only 10 dandelions sampled per quadrat; recommend 30+ for reliability.
• Control variables drifting — temperature in the room not held constant; use a thermostat.

CCEA tip

Pair every weakness with its improvement. "Heat is lost to the surroundings B1 so use a lid and lagging on the calorimeter B1" wins both marks; "the experiment lost heat" alone wins one.