You can't objectively measure a test unless you set a prediction for it beforehand
If you set a S.M.A.R.T. goal, it is easy to gauge success/failure of a test or project.
If you don't set a goal, it is easy to justify the results and string along a project for years before you understand if it is working.
S = Specific
M = Measurable
A = Achievable
R = Relevant
T = Time-bound