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LearnYBI11WBI16 · Unit 6: Practical Skills in Biology IIPractical skillsPlanning, analysis and evaluation
Practical skills · Subtopic 1 of 1

Planning, Analysis and Evaluation

Planning: state the independent, dependent and standardised (controlled) variables, how each is changed, measured or kept constant, and the range and intervals of the IV. Include a control (e.g. boiled enzyme) and repeats (at least 3). Assess risks. Statistical tests (with a null hypothesis of no difference/association): t-test compares two means; χ² compares observed with expected frequencies; Spearman's rank or Pearson's test correlation. If the calculated value is greater than the critical value at p = 0.05, reject the null hypothesis. Evaluation: validity (measures what it should, all other variables controlled), reliability/repeatability (consistent repeats), accuracy, precision; identify anomalies; suggest specific improvements (e.g. use a thermostatically controlled water bath).

Cambridge 9700 Paper 5Edexcel YBI11 WBI16AQA 7402 Paper 3≈ 50 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Plan investigations with variables, controls and repeats.
  • Choose the right statistical test.
  • Interpret test results using critical values.
  • Evaluate validity and suggest improvements.
Pause and think

You want to know if leaf length differs between shade and sun plants. Which test?

Section 2 of 14

Core idea

Start hereIs the difference real?

Two plant groups differ in mean height. Statistics tells you whether chance alone could explain it.

The big idea

Plan to change one variable and control the rest, then test results against a null hypothesis.

Choosing a statistical test

Match the test to the question.

Worked examplet-test

Means 12.0 and 10.0; variances 4.0 and 5.0; n = 10 each.

Goal
t.
Strategy
Difference ÷ SE of difference.
  1. 4.010+5.010=0.949\sqrt{\frac{4.0}{10}+\frac{5.0}{10}}=0.949
    Standard error of difference.
  2. t=2.00.949=2.11t=\frac{2.0}{0.949}=2.11
    Compare with 2.10 (df 18).
Answer2.11: significant at p = 0.05.
✓ CheckJust above critical ✓.

Summary

  • Variables and controls.
  • Choose the right test.
  • Validity and improvements.
Key words
Null hypothesis
Statement that there is no significant difference or correlation.
Validity
Whether an experiment measures what it intends to.
HistoryStudent's t-test. William Gosset devised the t-test in 1908 at the Guinness brewery and published under the pen name "Student".
Fun factLady tasting tea. Statistician Ronald Fisher designed an experiment to test a colleague's claim that she could tell whether milk or tea was poured first: the origin of the null hypothesis.
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