Important considerations in conducting statistical mediation analyses

An investigator wishes to examine mediation in a randomized control trial of the effectiveness of an intervention, which consists of a computerized decision aid for promoting colorectal cancer screening. Mediation is a naturally occurring process, and in any given instance, research investigators seek to ascertain whether it has occurred. In the case of a prevention intervention for a specific chain of events, mediation occurs (1) when the prevention intervention effects a change on a targeted intermediate condition: a mediator, for example, a person's intentions to get a colorectal screening examination; and (2) when, at a later point in time, this condition effects a change on a targeted outcome, for example, the actual behavior of getting a colorectal screening exam. Full mediation is said to occur when the effectiveness of the intervention on the targeted outcome only takes place through the intermediate condition and does not directly affect the targeted outcome. Partial mediation is said to occur when the intervention causes changes in both the intermediate condition and the targeted outcome. The assessment of mediation is important because conclusions about the efficacy of a public health intervention may depend on how these indirect influences are assessed.