18.3.1  Deciding what data to collect

The protocol should specify what outcomes and patient characteristics are to be analysed. However, before embarking on data collection it is sensible to ask the original investigators about what data are actually available. When deciding which variables to collect, it is often sensible to start by considering carefully what analyses are planned and what data will be needed to do them. This minimizes the possibility that essential information will not be sought or that unnecessary data will be collected. Understandably, investigators can get upset or suspicious if they have gone to the trouble of providing data that are not subsequently analysed and reported. 


Although in many cases it will be possible to collect specific variables for outcomes and characteristics as defined in the individual studies, it may be necessary to consider whether there are any data items for which further or constituent variables may be required. For instance, if studies have used different definitions of outcomes it may be desirable to redefine these for each patient in a consistent way across studies, and additional variables may be needed. For example, to redefine pre-eclampsia, data on systolic and diastolic blood pressure and proteinurea would need to be collected.