Quick questions on Unit 4 AoS 3 error analysis and data evaluation: VCE Biology

Random error. Unpredictable fluctuations: human reaction time when starting a stopwatch, small variations in mixing, small variations in lighting or temperature. Reduced by averaging over many replicates. Random error affects precision.

A higher-band evaluation quantifies error rather than describing it qualitatively. Examples:

Whether the method actually tests the hypothesis. A pH investigation with no buffering is not a valid test of enzyme activity at different pH (the pH drifts during the reaction). A measurement of "how fast plants grow" by counting leaves is not a valid measure of biomass (leaf area, dry mass, or height would be more valid).

The consistency of the measurement. If repeating the procedure produces similar results, the measurement is reliable. Reliability is improved by replication (multiple trials per condition), standardised method, and controlling variables.

How close repeated measurements are to one another. High precision means the values cluster tightly. Note that precision is independent of accuracy: a balance reading 50.000 g for a 25 g mass is precise (consistent to 0.001 g) but inaccurate.

How close a measurement is to the true value. Accuracy is improved by calibration (zeroing balances, calibrating thermometers, validating pH probes with reference buffers) and by minimising systematic bias.

Unpredictable fluctuations: human reaction time when starting a stopwatch, small variations in mixing, small variations in lighting or temperature. Reduced by averaging over many replicates. Random error affects precision.

A consistent bias in one direction: an uncalibrated balance reading 0.5 g high, a stopwatch that runs slow, a pH probe that reads 0.2 units high. Reduced by calibration and by checking instruments against a known standard. Systematic error affects accuracy.

A one-off mistake: misreading the instrument, contaminating a sample, transposing a number when recording. Reduced by carefully recording at the time of measurement and by double-checking. Gross errors usually appear as outliers that should be investigated, not discarded silently.

Distinguish between validity and reliability with a biological example of each. [4 marks]

A student investigates the effect of light intensity on the rate of photosynthesis using oxygen bubble counting from elodea. Identify two sources of systematic error and one source of random error, and propose one improvement for each. [6 marks]

A student reports peak enzyme activity at pH 7 based on three trials per pH. The pH 6 mean is higher than expected and the standard deviation is large. Critique the data and recommend next steps.