Question

Professor Newberg decided to study the social activity level of senior citizens, so he visited a local retirement village and invited residents to participate in a lengthy survey. Professor Newberg's survey results are probably invalid because he did not use:

Answer 1

Answer: A random selection of participants.

In order for professor Newberg's survey to be considered valid, he should have had a random selection of participants, instead.  By doing so, all members would have an equal chance of being included in the study.