For a single trial, if the outcome in each group is expressed as a rate, the relative risk is the ratio of the rate in the experimental group to that in the control group. The odds ratio describes the odds of a patient in the experimental group having the outcome relative to a patient in the control group. For rare outcomes, the relative and the odds ratio are virtually interchangeable, but for more common outcomes they can be very different. Systematic reviews tend to combine trials of varying follow-up periods, which could make an NNT uninterpretable. They can be calculated when the follow-up periods are similar, but these should be stated in the text.
Some outcomes take the form of a count.
Examples include number of hospital admissions, number of occurrences of a particular adverse event. These should always be defined with respect to a time period. The difference between the groups can then be summarised, for example by a ratio of rates. Because CF is a chronic disease, data are longitudinal in nature, with the potential for many repeated measures. There is also a choice as to whether to look at raw values, of say FEV1, or change, expressed either as a difference or a percentage.