Ow effective are the departmental select committees at scrutinising executive action?

The question above comes under Parliament,Politics and Policy Making. Only please talk about the departmental select committees about what they are and what they do and then begin to examine their relationship with the executive and then look at how it scrutinises the executive and please have plenty of examples of times where it has succeeded and not succeeeded.And is it effective at scrutinising the executive as the conclusion. Also I require just at the end separately a page on its own listing all the summary points of the essay please!
Make sure it has footnotes and referencing withing the essay.