How can I estimate how long my trials will take?

When estimating how long your trials will take, it may help to consider the following:

1. How long will it take to conduct the intervention itself?
2. What is the volume of the stream of potential research participants?  In some studies, such as school-based projects, potential research participants may be readily available in large quantities all at once (e.g. every fall), making it possible to conduct experiments relatively quickly.  In other studies, participants may “trickle” in slowly, and there may not be much that can be done to increase the volume of the trickle.  In this case, the length of time it will take to recruit enough participants may be a limiting factor.
3. How far off in the future is the horizon on the outcome?  Some interventions can be evaluated fairly quickly; for example, whether or not a smoking cessation intervention has been successful can often be evaluated in a month or less.  By contrast, other interventions can take much longer to evaluate; an intervention to prevent a second heart attack may require observing participants for years before conclusions can reasonably be drawn.