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A scatter plot is illustrated in Figure 2.8 for the baseline corn size against the three‐month follow‐up corn size for 181 patients with corns on their feet. There appears to be some association between baseline and three‐month corn size in this sample with larger baseline corn sizes associated with larger three‐month corn sizes and vice versa. There are numerous overlapping data points in this scatterplot with several patients having the same combination of baseline and three‐month corn sizes. Overlapping or overplotting can occur when a continuous measurement, in this example corn size, is rounded to some convenient unit (e.g. the nearest mm). To prevent overlapping data points in Figure 2.8, we have added a small random noise to the data called jittering. Jittering is the act of adding random noise to data in order to prevent overplotting in statistical graphs.

Figure 2.8 Scatter plot of baseline corn size by corn size at a three month follow‐up for 181 patients with corns.

(Source: data from Farndon et al. 2013).

It is likely that baseline corn size will have an influence on corn size at three months, but vice versa cannot be the case. In this case, if one variable, x, (baseline corn size) could cause the other, y, (three‐month corn size) then it is usual to plot the x variable on the horizontal axis and the y variable on the vertical axis.

In contrast, if we were interested in the relationship between baseline corn size and height of the patient then either variable could cause or influence the other. In this example it would be immaterial which variable (corn size or height) is plotted on which axis.