Table 3.9 Results of randomised controlled trial in primary care in patients with venous leg ulcers to compare a new specially impregnated bandage, called ‘Band aid’, with usual care.
| Leg ulcer completely healed | Group | |
|---|---|---|
| Band‐aid intervention | Usual care control | |
| Yes, healed | 147 | 123 |
| No, not healed | 63 | 82 |
| Total | 210 | 205 |
From the data in Table 3.9:
3.2 Points When Reading the Literature
1 Is the number of subjects involved clearly stated in the table?
2 Are the row and columns in the table clearly labelled?
3 Do the titles adequately describe the contents of the table?In tables:
4 If percentages are shown, is it clear whether they add across rows or down columns? For example in Table 3.4 it is clear the percentages total down the columns, not across the rows.
5 Percentages should not have decimal places if the number of subjects in total is less than 100.Summary statistics:
6 If a relative risk is quoted, what is the ARD? Is this a very small number? Beware of reports that only quote relative risks and give no hint of the absolute risk!
7 If an odds ratio is quoted, is it a reasonable approximation to the relative risk? (Ask what the size of the risk in the two groups are).
3.3 Exercises
Table 3.9 shows the results of randomised controlled trial in primary care in patients with venous leg ulcers to compare a new specially impregnated bandage, called ‘Band aid’, with usual care. Usual care will be treatment by district nurses with standard bandages and wound dressings. The primary outcome for the study will be whether or not the index or reference leg ulcer has completely healed at a 12‐month post‐randomisation.
1 3.1 What proportion of patients in the Band‐aid group had a completely healed leg ulcer at 12 months?0.300.400.600.650.70
2 3.2 What proportion of patients had a completely healed leg ulcer at 12 months in the control group?0.300.400.600.650.70
3 3.3 What is the difference in response (leg ulcer healing rates at 12 months) between the Band‐aid and control groups?−0.10−0.050.000.050.10
4 3.4 Calculate the number of people needed to be treated with Band‐aid dressing in order for an additional person to have a completely healed leg ulcer at 12 months compared to people usual care?6 78810
5 3.5 What is the relative risk for the leg ulcer healing at 12 months in the Band‐aid group compared to the control group?0.640.861.171.562.33
6 3.6 What is the relative risk for the leg ulcer healing at 12 months in the control group compared to the Band‐aid group?0.640.861.171.562.33
7 3.7 What are the odds for the leg ulcer healing at 12 months in the Band‐aid group?0.640.861.501.562.33
8 3.8 What are the odds for the leg ulcer healing at 12 months in the control group?0.640.861.501.562.33
9 3.9 Calculate the odds ratio for the leg ulcer healing at 12 months in the Band‐aid group compared to the control group?0.640.861.171.562.33
10 3.10 Calculate the odds ratio for the leg ulcer healing at 12 months in the control group compared to the band‐aid group?0.640.861.171.562.33
4 Probability and Distributions
2 4.2 The Binomial Distribution
3 4.3 The Poisson Distribution
4 4.4 Probability for Continuous Outcomes
8 4.8 Points When Reading the Literature
Summary
Probability is defined in terms of either the long‐term frequency of events, as model based or as a subjective measure of the certainty of an event happening. Examples of each type are given. The concepts of independent events and mutually exclusive events are discussed. Several theoretical statistical distributions, such as the Binomial, Poisson and Normal are described. The properties of the Normal distribution and its importance are stressed and its use in calculating reference intervals is also discussed.
4.1 Types of Probability
There are a number of ways of looking at probability and we describe three as the ‘frequency’, ‘model‐based’ and ‘subjective’ approaches as shown in Figure 4.1.
Figure 4.1 Three types of probability.
We all have an intuitive feel for probability but it is important to distinguish between probabilities applied to single individuals and probabilities applied