Defining the exact meaning of the variables involves two things (maybe even deciding to use the same standard). First, all parties within the collaboration agree to use the same units (e.g., for
length one could decide to use
cm instead of
mm). For categorical variables this would mean deciding on the value sets. Second, the collaboration would have to make sure that all variables are actually comparable. For example, make sure that all sites measured
blood pressure in the same way (e.g., lying down).