Farndon et al. (2013) reports a randomised controlled trial that investigated the effectiveness of salicylic acid plasters compared with usual scalpel debridement for treatment of foot corns. As we have already mentioned one categorical variable recorded was the centre where each trial participant was treated. Trial participants were treated at one of seven centres and the corresponding categories as displayed in Table 2.2. The first column shows category (treatment centre) names, whilst the second shows the number of individuals in each category together with its percentage contribution to the total. Since the total sample size is more than 100 we have reported the percentages to one decimal place. Table 2.2 clearly shows that the majority (54.5%) of patients were treated at the ‘Central’ treatment centre.
Table 2.2 Treatment centre for 202 patients with corns who were recruited to a randomised control trial of the effectiveness of salicylic acid plasters compared with ‘usual’ scalpel debridement for the treatment of corns
(Source: data from Farndon et al. 2013).
| Treatment centre | Frequency | Percentage |
|---|---|---|
| Central | 110 | 54.5% |
| Manor | 33 | 16.3% |
| Jordanthorpe | 24 | 11.9% |
| Limbrick | 9 | 4.5% |
| Firth Park | 11 | 5.4% |
| Huddersfield | 9 | 4.5% |
| Darnall | 6 | 3.0% |
| Total | 202 | 100.0 |
In addition to tabulating each variable separately, we might be interested in whether the distribution of patients across each centre is the same for each randomised group. Table 2.3 shows the distribution of the number of patients treated at centre by randomised group; in this case it can be said that treatment centre has been cross‐tabulated with randomised group. Table 2.3 is an example of a contingency table with seven rows (representing treatment centre) and two columns (randomised group). Note that we are interested in the distribution of patients across the seven centres in each randomised group (to see whether or not we have similar numbers of patients randomised to each treatment within each centre), and so the percentages add to 100 down each column, rather than across the rows. In this example since we have 101 and 101 patients in each randomised group the percentages are almost the same as the raw counts. However, for most studies you are unlikely to have exactly 100 participants in each group!
Table 2.3 Cross‐tabulation of treatment centre by randomised group for 202 patients with corns who were recruited to a randomised control trial of the effectiveness of salicylic acid plasters compared with ‘usual’ scalpel debridement for the treatment of corns
(Source: data from Farndon et al. 2013).
| Randomised group | |||
|---|---|---|---|
| Corn plaster | Scalpel | All | |
| n (%) | n (%) | n (%) | |
| Central | 58 (57) | 52 (52) | 110 (54.5) |
| Manor | 13 (13) | 20 (20) | 33 (16.3) |
| Jordanthorpe | 10 (10) | 14 (14) | 24 (11.9) |
| Limbrick | 3 (3) | 6 (6) | 9 (4.5) |
| Firth Park | 7 (7) | 4 (4) | 11 (5.4) |
| Huddersfield | 5 (5) | 4 (4) | 9 (4.5) |
| Darnall | 5 (5) | 1 (1) | 6 (3.0) |
| Total | 101 (100) | 101 (100) | 202 (100) |
Labelling Binary Outcomes
For binary data it is common to call the outcome ‘an event’ or ‘a non‐event’. For example, having your corn healed and resolved after three months of treatment may be an ‘event’. We often score an ‘event’ as 1 and a ‘non‐event’ as 0. These may also be referred to as a ‘positive’ or ‘negative’ outcome or ‘success’ and ‘failure’. It is important to realise that these terms are merely labels and the main outcome of interest might be a success in one context and a failure in another. Thus, in a study of a potentially lethal disease the outcome might be death, whereas in a disease that can be cured it might be being alive.
2.3 Displaying Categorical Data
Two methods of displaying categorical data are a bar chart or a pie chart. Figure 2.2 shows in a bar chart the recruiting centres of 202 patients with foot corns treated in the trial of Farndon et al. (2013). Along the horizontal axis are the different treatment centre categories whilst on the vertical axis is the percentage. Each bar represents the percentage of the total patient population in that category. For example, it can be seen that the percentage of participants who were treated in the Central centre was about 55%.
Figure 2.2 Bar chart showing where 202 patients with corns were treated
(Source: Farndon et al. 2013).