One of the many problems with medical publications is that researchers will choose which outcomes to report based on their statistical significance rather than their clinical importance. This can seriously bias your results. You can easily avoid this potential bias by specifying your primary and secondary outcome measures prior to data collection. Apparently, though, some researchers will change their minds after designating these outcome variables and fail to report on some of the outcomes and/or add new outcomes that were not specified prior to data analysis. How often does this occur? A group of scientists at the Centre for Evidence-Based Medicine at the University of Oxford are trying to find out.
Goldacre B, Dale A, Milosevic I, Mahtani K, Powell-Smith A, Drysdale H, Hartley P, Slade E, Heneghan C. The COMPare Project. Available in html format.
You can find an earlier version of this page on my blog.