Graph showing normal distribution

What is normal?

Many things follow a normal distribution

Datasets can be spread out in many different ways. The majority of the data can sit above the mean or below it. In many datasets, however – particularly large ones – the data points seem to settle equally on either side of the mean.

Plotted on a graph, the shape of the distribution resembles a bell and so is sometimes called a ‘bell curve’. This is also called a normal distribution). Standard deviation is a measure of how spread out the numbers are around the mean. If a dataset follows a normal distribution, approximately 68 per cent of the data will fall between one standard deviation on either side of the mean. Around 95 per cent will fall within two standard deviations on either side. In such circumstances, the mean, median and mode of the data are all equal.

There are many everyday biological examples that follow a roughly normal distribution, including blood pressure, height and foot length. Along with these examples, you could also try looking at stalk height in daisies, the length of holly leaves or the diameter of lichens (commonly found on gravestones).

About this resource

This resource was first published in ‘Number Crunching’ in June 2013.

Statistics and maths, Medicine
Number Crunching
Education levels:
16–19, Continuing professional development, Undergraduate