Sandra’s ability to improve her study behavior is best considered an example of "shaping".
Shaping includes an ascertained reinforcement of an "objective behavior". It utilizes operant conditioning standards to prepare a subject by compensating legitimate conduct and debilitating inappropriate conduct.
Shaping includes efficiently fortifying progressive approximations to a desired conduct. In this case, Sandra fortifies her progressive approximations (examining for 5 minutes) of the desired conduct (considering for 15 minutes without diversion).