Which of the following is a characteristic of contamination? O This refers to exlusion in a test of a group of respondents who are not normally there. This occurs when buyers from outside the test area come into the area to purchase the product being tested. O This can improve the validity of the experiment. O This refers to systematic differences between the test group and the control group due to a biased selection process. This refers to changes in measurement instruments (e.g., interviewers or observers) that might affect measurements.