I was reading the book The Unwritten Rules of PhD Research by Marian Petre when I came across the following idiom:

... anyone with an armchair and reasonable general knowledge can usually find several reasons for querying the representativeness of a sample without much effort. These scruples tend to be slaughtered on the altar of expediency the moment that the questionnaires go into the post (questionnaires are a favourite method for collecting large and dubious data sets)...

A google search shows this is not an extremely common phrase. What does it mean?