All subjects · Psychology · WPS02 Biological Psychology, Learning Theories and Development

Descriptive statistics (List A)

Edexcel IAL Psychology · WPS02 Biological Psychology, Learning Theories and Development

List A from Topic A applies to Unit 2: measures of central tendency and dispersion, tables, graphs, percentages and distributions (spec 3.2.7 and 4.2.5).

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Central tendency

Mean: add the scores and divide by how many. Uses all the data, but is distorted by extreme scores. Interval/ratio data.
Median: the middle score when ordered (average of the two middle scores if there is an even number). Not affected by extreme scores. Ordinal data or above.
Mode: the most common score. The only average for nominal data; there may be more than one.

Dispersion

Range: highest minus lowest. Quick, but affected by extreme scores.
Standard deviation (SD): the average spread of scores around the mean. A large SD means scores are spread out. The exam gives the sample estimate formula: SD = √(Σ(x - mean)² / (N - 1)).

Steps: find the mean; subtract it from each score; square each difference; add them; divide by N - 1; take the square root. Show your working: examiners note that working can earn credit.

Tables, graphs and distributions

Frequency tables count how often each score or category occurs; summary tables show averages and spread for each condition.
Bar charts: separate categories (nominal data), bars not touching.
Histograms: continuous data, bars touching, area shows frequency.
Scatter diagrams: correlations.
Normal distribution: symmetrical bell curve, mean = median = mode.
Positive skew: tail to the right (a few high scores), mean pulled above the median.
Negative skew: tail to the left, mean below the median.
Percentages, ratios and fractions: e.g. 12 out of 40 = 30% = 3:7 (yes:no) = 3/10.

Worked example

Aggression scores for 10 children: 4, 7, 6, 9, 5, 7, 12, 7, 6, 8. Find the mean, median, mode and range.

Mean = 71 / 10 = 7.1.
Ordered: 4, 5, 6, 6, 7, 7, 7, 8, 9, 12. Median = (7 + 7) / 2 = 7.
Mode = 7 (appears 3 times).
Range = 12 - 4 = 8.

Answer: Mean 7.1, median 7, mode 7, range 8.

Key idea: Mean (all data, skewed by outliers), median (middle), mode (most common). Range and SD show spread. Bar chart for categories, histogram for continuous data.

Check yourself

Find the mean of 4, 7, 6, 9, 5, 7, 12, 7, 6, 8.
Show answer
7.1. 71 / 10 = 7.1.
Find the median of 4, 7, 6, 9, 5, 7, 12, 7, 6, 8.
Show answer
7.0. Middle two of the ordered list are 7 and 7.
Calculate the standard deviation (sample estimate, divide by N - 1) of 3, 5, 6, 8, 8 to 2 decimal places.
Show answer
2.12. Mean = 6. Squared deviations add to 18. Divide by 4 = 4.5. Square root = 2.12.
In an observation, 18 of 48 drivers broke the speed limit. What percentage is that?
Show answer
37.5 %. 18 / 48 x 100 = 37.5%.
Which average is the only one suitable for nominal data?
- Mean
- Mode
- Median
- Standard deviation
Show answer
Mode. Categories can only be counted.
A few very high sleep times pull the mean above the median. The distribution is
- negatively skewed
- normal
- bimodal by definition
- positively skewed
Show answer
positively skewed. Tail to the right, mean above median.
Which graph suits continuous data such as hours of sleep grouped into intervals?
- Bar chart
- Histogram
- Pie chart of names
- Scatter diagram
Show answer
Histogram. Histograms show continuous data with touching bars.

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Last updated 8 November 2026. Spotted a mistake? Tell us.