Answer:
Attrition
Step-by-step explanation:
Attrition: The term attrition refers to the process of a group getting smaller in number because members of the group are dropping out. Due to dropping out of the member of a group for different reasons in a research study, the reliability is being threatened.
In other words, if cases of a research study are getting lost over time then it will lead to attrition.
Key feature: Progressive loss of data in a research study or the wearing away.
The kind of problem Gerard has encountered in his study is attrition.