The blue lines show deciles. There are nine deciles and they divide the sorted data into ten equal parts.

The thickest blue line shows the median. The median is the fifth decile. Half of all organisations are above the median; half are below.

The red line, if present, highlights a single organisation.

Understanding the median

To understand deciles, it helps to understand the median.

Imagine we have prescribing data for nine practices for one month. Let's order the prescribing data from smallest to largest.

[4, 16, 39, 40, 43, 56, 58, 67, 97]

The median splits the values into the lower half and the higher half. In this case, the median is 43. The four values in the lower half are less than the median. The four values in the higher half are greater than the median.

Calculating the median isn't always as straightforward. What should we do when there are ten practices, for example?

[4, 16, 39, 40, 43, 44, 56, 58, 67, 97]

In this case, two values separate the lower half from the higher half: 43 and 44. In OpenPrescribing, we use the value that's half-way between these two values as the median: 43.5.

The median is the fifth decile. With the other deciles, we still split the values into a lower part and a higher part. However, the number of values in each part is different. For example, 20% of values are less than the second decile and 80% of values are greater than the second decile.

Understanding a single organisation

If you've chosen to highlight a single organisation, then you may be wondering how it relates to the median. If its line is below the median, then its prescribing is less than the median. Similarly, if its line is above the median, then its prescribing is greater than the median. The same is true for the other deciles.