Exposure or outcome?
Let us consider an example. Gårdvik et al. report results from a follow-up study of individuals three years after their period as outpatients in a mental health clinic for children and youth (2). Of 464 persons, 256 were women, 109 of whom were diagnosed with anxiety at the time of follow-up, while 147 were not. Of 208 men, a total of 43 were diagnosed with anxiety, while 165 were not (Table 2 in (2)). These figures can be cross-tabulated as shown in Table 1. Here, the percentages of persons with and without a diagnosis within each gender are shown first, i.e. percentages in the rows of the tabular presentation. In this study, 43 % of the women and 21 % of the men were diagnosed with anxiety.
Table 1
Contingency table of persons diagnosed and not diagnosed with anxiety, by gender (from Table 2 in (2)). Percentages within each gender are placed to the right of the counts, while percentages within each diagnosis group are placed below the counts.
| | Diagnosed with anxiety | No diagnosis | Total |
|---|
| Women | 109 (43 %)
(72 %) | 147 (57 %)
(47 %) | 256 (100 %) |
| Men | 43 (21 %)
(28 %) | 165 (79 %)
(53 %) | 208 (100 %) |
| Total | 152
(100 %) | 312
(100 %) | 464 |
Could it alternatively be relevant to show percentages within each column? These percentages are also shown in Table 1: Among those who had the diagnosis, 72 % were women, and among those who did not, 47 % were women.
The two alternatives elucidate two different questions: a) What proportion of the men and women in the study have the diagnosis?; and b) What are the proportions of men and women in the groups with and without the diagnosis?
In this case, it is most natural to think of gender as exposure and the diagnosis of anxiety as outcome, rather than the opposite, and a) will be the natural question to ask. It will thus be appropriate to report percentages within each gender, i.e. in the rows, as shown in the table. Table 1 also includes 100 % for the total, to make it clear to the reader whether the percentages are reported in the rows or the columns.