When is a screening experiment appropriate

The screening problem is often encountered in organic synthesis. Some examples will illustrate this  

This will also be an impetus to probe on pertinent details to gain an understanding at the mechanistic level. Any suggested reaction mechanism must be able to account for the observation that some variables are important, while others are not. 

The most profitable way to do this will obviously be to adjust the most influential variables. There would be no point in trying to adjust factors which have a negligible influence on the result. 

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