# 5: Trial size

- Page ID
- 13164

- 5.1: Introduction to trial size
- One of the most important factors to consider in the design of an intervention trial (or indeed in the design of any epidemiological study) is the choice of an appropriate trial size to answer the research question. Trials that are too small may fail to detect important effects of an intervention on the outcomes of interest or may estimate those effects too imprecisely.

- 5.2: Criteria for determining trial size
- To select the appropriate sample size, it is necessary to decide how much sampling error in the estimate of the effect of the intervention is acceptable and to select the sample size to achieve this precision.

- 5.3: Size to give adequate precision
- This section describes how the trial size is determined if the aim is to obtain an estimate of the outcome of an intervention with a specified level of precision.

- 5.4: Size to give adequate power
- The alternative approach to setting trial size is based upon selecting the trial size to achieve a specified power. In order to do this, the following must be specified:

- 5.5: More complex designs
- Sections 3 and 4 considered the simplest situation where the two groups to be com- pared are of equal size. Sometimes, there may be reasons for wishing to allocate more individuals to one group than to the other. For example, if an experimental drug is very expensive, it may be desired to minimize the number of patients allocated to the

- 5.6: Interventions allocated to groups
- The methods described in Sections 3 to 5 all assume that individuals are to be the units of allocation. In other words, the trial groups will be constructed effectively by making a complete list of the individuals available for the trial and randomly selecting which individuals are to be allocated to each trial group. As explained in Chapter 4, Section 4, however, many field trials are not organized in this way. Instead, groups of individuals are allocated to the interventions under study. These

- 5.7: Other factors influencing choice of trial size
- It is sometimes desirable to incorporate interim analyses into the trial plan, involving review of the results at (say) 6-monthly or annual intervals.

- 5.8: The consequences of trials that are too small
- The methods outlined in this chapter for selecting an adequate sample size have been available for many years, but it is probably not an exaggeration to state that the majority of intervention trials are much too small. Although there is an increasing awareness of the need to enrol a large enough sample, this chapter is concluded by discussing the consequences of choosing a sample size that is too small.

- 5.9: Computer software for sample size calculations
- Most of the formulae given in this chapter are simple enough to do by hand, with the aid of a simple calculator. However, computer software is also available to carry out some of these calculations.