S1 exam technique

Accuracy

• Give inexact answers to 3 significant figures unless the question says otherwise.
• Use exact values, or full calculator values, in working; round only at the end. A rounded mean in a variance calculation can cost the accuracy mark.
• Take z values from the percentage points table to 4 d.p. (1.2816, not 1.28).

Show your working

When a question says calculator-only solutions are not acceptable, or asks you to 'show that', every step must be visible. Write the numerical expression for Sxx, Syy, Sxy and the standardisation (x - μ)/σ before giving the value. A correct conditional probability with no working can score zero.

Context

Interpretation marks need the context: for a PMCC, say what the correlation means for the two named variables; for comparisons, name the statistic, give its value and say what it means ('the median running time is longer'). 'Strong positive correlation' alone is not enough.

Check your answers

Probabilities must lie between 0 and 1. Predictions should be sensible (no negative masses). Read whether a question wants P(X < a) or P(X > a), and sketch the normal curve every time.

Worked example

15 values have Σx = 202 and Σx² = 2946. Show why the mean should not be rounded before finding the standard deviation.

1. Exact: x̄ = 202/15 = 13.4666..., σ² = 196.4 - 13.4666...² = 15.0489
2. Rounded: using 13.5 gives σ² = 196.4 - 182.25 = 14.15

Answer: σ = 3.88 (exact) against 3.76 (rounded mean): the rounded answer loses the accuracy mark.

Check yourself

1. 15 values have Σx = 202 and Σx² = 2946. Find the standard deviation, keeping the exact mean in your working. Give your answer to 3 significant figures.
   Show answer

   3.88. x̄ = 202/15; σ² = 2946/15 - (202/15)² = 15.0489; σ = 3.88.

2. Write 0.106543 to 3 significant figures.
   Show answer

   0.107. The first three significant figures are 1, 0, 6; the next digit is 5, so round up: 0.107.

3. The S1 paper has 75 marks in 90 minutes. About how many minutes should a 10-mark question take?
   Show answer

   12. 90/75 = 1.2 minutes per mark, so 12 minutes.

4. Which z value should you use for P(Z > z) = 0.1 in an S1 answer?
   • 1.28
   • 1.3
   • 1.2816
   • 0.1
   Show answer

   1.2816. Examiners expect percentage points to 4 d.p.

5. Which interpretation of r = 0.93 between hours of sunshine and ice-cream sales earns the mark?
   • Days with more sunshine tend to have higher ice-cream sales
   • There is strong positive correlation
   • r is close to 1
   • The data are reliable
   Show answer

   Days with more sunshine tend to have higher ice-cream sales. It refers to the named variables in context.

6. Your answer to a probability question is 1.08. What should you do?
   • Round it to 1
   • Give it as 108%
   • Recheck the working, as a probability cannot exceed 1
   • Subtract it from 1
   Show answer

   Recheck the working, as a probability cannot exceed 1. Probabilities lie between 0 and 1; 1.08 signals an error.

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Sources

• Pearson Edexcel International Advanced Level in Mathematics specification (issue 3), Unit S1
• Pearson Edexcel IAL Statistics S1 (WST01) examiners' report, October 2024

Last updated 7 November 2026. Spotted a mistake? Tell us.