The chapter on mathematical concepts deals with graphs as illustrations of relationships between variables. The chapter on descriptive statistics introduces statistical terms.

Diagrams

Diagrams are used to illustrate statistical data (see the chapter on descriptive statistics for explanation of terms).

The pie diagram is the simplest form. It is often used to illustrate data collected from audit. It is useful if there are several categories (portions) of a whole, for instance age groups, different surgical specialties, items of expenditure in budget, etc. The pie diagram gives an immediate idea of how the whole is divided up. Figure 36 shows all patients who experienced nausea after surgery according to the type of operation.

The histogram is best used to show counts – integer numbers of patients, objects or days. Bands or columns are used for the illustration; although each number has only one dimension, plotted against the y-axis, the second dimension – the width of the band – makes the visual distinction between groups easier and distinguishes a histogram from a line graph. Figure 37 shows the comparison of the incidence of postoperative nausea among men and women. Measurements of continuously variable data are best represented by scatter diagram (see below) although the histogram is often used in this situation.

The frequency histogram illustrates the distribution of different values of a measured variable in a certain sample or population. Figure 38 shows the distribution of hypothetical laboratory measurements of fasting blood glucose concentration in all employees of a hospital.