What do you do with the basic summary statistics that convey a general idea of how a set of numbers is distributed? Generally, when presenting your results, you pick a few of the most useful summary statistics and arrange them in a concise way. Many biostatistical reports select N, mean, SD, median, minimum, and maximum, and arrange them something like this:
mean ± SD (N)
median (minimum – maximum)
For example, the data (84, 84, 89, 91, 110, 114, and 116), the preceding arrangement looks like this:
98.3 ± 14.4 (7)
91 (84 –116)
The real utility of this kind of compact summary is that you can place it in each cell of a table to show changes over time and between groups. For example, systolic blood pressure measurements, before and after treatment with a hypertension drug or a placebo, can be summarized very concisely.
| Drug | Placebo | |
|---|---|---|
| Before Treatment | 138.7 ± 10.3 (40) 139.5 (117 – 161) | 141.0 ± 10.8 (40) 143.5 (111 – 160) |
| After Treatment | 121.1 ± 13.9 (40) 121.5 (85 – 154) | 141.0± 15.4 (40) 142.5 (100 – 166) |
| Change | –17.6 ± 8.0 (40) –17.5 (–34 – 4) | –0.1 ± 9.9 (40) 1.5 (–25 – 18) |
This table shows that the drug tended to lower blood pressure by about 18 millimeters of mercury (mmHg), from 139 to 121, whereas the placebo produced no noticeable change in blood pressure (it stayed around 141 mmHg). All that's missing to make this table really informative are some p values to indicate the significance of the changes over time within each group and of the differences between the groups.