Eduqas A Level Psychology Levels of Measurement, Graphs and Descriptive Statistics Exam Questions

About Eduqas A Level Psychology Levels of Measurement, Graphs and Descriptive Statistics

This topic in Component 2 is about summarising and presenting data. You must identify the level of measurement of a variable, choose and draw the right graph, recognise normal and skewed distributions, and calculate and interpret measures of central tendency and dispersion. These skills feed directly into choosing inferential tests and into the novel scenario questions, which often supply a table of results. Examiners test calculations, so be ready to work out a mean, median, mode, range or percentage, and to explain what a standard deviation tells you about the spread of scores in each condition.

What you need to know

• Levels of measurement Nominal data are categories (for example yes or no). Ordinal data can be ranked but intervals are not equal (for example ratings). Interval data have equal intervals on a scale, and ratio data also have a true zero (for example time in seconds).
• Graphs Bar charts show nominal or category data with gaps between bars. Histograms show continuous data with no gaps. Scatter diagrams show correlations. Line graphs show changes, and pie charts show proportions of a whole.
• Distributions In a normal distribution the mean, median and mode are equal and the curve is symmetrical. In a positive skew the tail is to the right and the mean is higher than the median; in a negative skew the tail is to the left and the mean is lower.
• Central tendency The mean uses all scores but is affected by extreme values. The median is the middle score and is not distorted by outliers. The mode is the most frequent score and is the only measure suitable for nominal data.
• Dispersion The range is the highest minus the lowest score (some methods add 1). The standard deviation shows the average spread of scores around the mean; a larger SD means scores are more spread out.

Common mistakes in Levels of Measurement, Graphs and Descriptive Statistics

• Drawing a histogram for categorical data. Use a bar chart with gaps for categories; histograms are for continuous data.
• Saying a large standard deviation means high scores. It means scores are widely spread around the mean.
• Getting skew directions wrong. Skew is named after the tail: positive skew has the tail to the right.
• Forgetting to order the data before finding the median. Put scores in order first, and average the middle two when there is an even number.
• Using the mean for nominal data. Only the mode can be used for categories.

Try three Levels of Measurement, Graphs and Descriptive Statistics exam-style questions