Measurements in Clinical Trials

Efficacy is often the main objective of the trial and is usually the aspect of highest interest. This part of the analysis is based on pre-defined ‘endpoints. These are specific measurements related to the illness in question that have been specified in advance in the protocol (the document which describes in detail how the trial is going to be performed).

Endpoints in general can be categorised as:

• ‘Hard’ endpoints – those that take the form of numerical facts with intrinsic clinical importance. For example, how long the participant survived or what proportion of participants recovered from an infection.
  
  
• ‘Soft’ endpoints – those which are potentially influenced by the measurement process or with questionable reproducibility. For example, a quality-of-life questionnaire or the description of the participant’s mood at a given moment. In order to be analysed statistically, soft endpoints have to be converted into a numerical format. This process can be controversial as it is subjective and potentially open to inconsistencies.
  
  
• ‘Surrogate’ endpoints – those that are not in themselves part of the patient’s experience of the illness, but may be closely related to it. For example, the results of laboratory tests. 

Often, choosing which endpoints to use depends heavily on the nature of the illness being studied. Cancer offers obvious hard endpoints in the form of survival, whereas evaluation of depression. Other illnesses, such as diabetes, are associated with well-established surrogate endpoints such as blood sugar levels.