Improvement of the measurement system to the point where the errors in it are small compared to the errors in the system under test is vital. Selecting a theory Prediction, and the rules of operational meaning, are only one side of the scientific process. They tell us how to test, and so improve, a theory. Test the simplest theory first. A simple theory is also easier to communicate, to disprove, and so improve.
Every theory has a limited range, or domain within which it applies.
You do not understand a theory until you know when it will not work. Study extreme cases to find the limits and increase knowledge. This is because a theory which links previously separate things, like Newton's theory connecting falling apples with the motion of planets, generates new opportunities for testing, and so leads to faster progress. This still leaves a wide choice of ideas that could be tested. We can avoid wasting time by learning to ask the right question.