She admitted the invalid assumption of classroom differentiation, and lost a year of doctoral research. Maybe she should have written her thesis on the methodological difficulties in measuring many of our educational variables. She certainly did have the data for that. Informed respondents are those who have some relationship to the research problem and who are consequently more likely to be informed on the related issues. If you are studying crime, corrections officers, police, social workers, teachers, juvenile justice workers are all likely to have had experience with crime in the course of their professional work.
Most professionals are glad to help in such endeavors.
Increasingly the Web provides such a forum. You'll need to scroll down almost to the bottom of the page to find the call for help in gathering information on Assisting Victims of Campus Crime. Not only does the Web site offer you a forum from which to request help in gathering data. Its forms provide the means to do so.