Using key opinion leaders may affect the type of sites that is selected. For example the academic or non-academic sites might be more willing to follow the opinion leader, and if the type of site is an effect modifier, it may affect the generalisability of the results. However, the exact consequence is often difficult to predict. Key opinion leaders will usually request a fee, so costs will increase but recruitment might be easier with help of a key opinion leader. Net costs will depend on the effects.