Читать книгу Medical Statistics - David Machin - Страница 67

In any graph there are clearly certain items that are important. For example, scales should be labelled clearly with appropriate dimensions added. The plotting symbols are also important; a graph is used to give an impression of pattern in the data, so bold and relatively large plotting symbols are desirable. This is particularly important if it is to be reduced for publication purposes or presented as a slide in a talk.

A graph should never include too much clutter; for example, many overlapping groups each with a different symbol. In such a case it is usually preferable to give a series of graphs, albeit smaller, in several panels. The choice of scales for the axes will depend on the particular data set. If transformations of the axes are used, for example, plotting on a log scale, it is usually better to mark the axes using the original units as this will be more readily understood by the reader. Breaks in scales should be avoided. If breaks are unavoidable under no circumstances must points on either side of a break be joined. If both axes have the same units, then use the same scale for each. If this cannot be done easily, it is sensible to indicate the line of equality, perhaps faintly in the figure. False impressions of trend, or lack of it, in a time plot can sometimes be introduced by omitting the zero point of the vertical axis. This may falsely make a mild trend, for example a change from 101 to 105, into an apparently strong trend (seemingly as though from 1 to 5). There must always be a compromise between clarity of reproduction that is filling the space available with data points and clarity of message. Appropriate measures of variability should also be included. One such is to indicate the range of values covered by two standard deviations each side of a plotted mean.

It is important to distinguish between a bar chart and a histogram. Bar charts display counts in mutually exclusive categories, and so the bars should have spaces between them. Histograms show the distribution of a continuous variable and so should not have spaces between the bars. It is not acceptable to use a bar‐chart to display a mean with standard error bars (see Chapter 6). These should be indicated with a data point surrounded with errors bars, or better still a 95% confidence interval.

With currently available graphics software one can now perform extensive exploration of the data, not only to determine more carefully their structure, but also to find the best means of summary and presentation. This is usually worth considerable effort.