Suppose you are interested in how introverts and extroverts tend to interact with strangers. You hypothesize the introverts will be less comfortable and should display more nervous movements. You use a camera to record structured interviews with both introverts and extroverts, tracking head and body movements of all participants to measure their level of comfort during the activity. You later wonder if eye movement can help indicate comfort level. However, the camera you used couldn't track these fine movements, so you had to leave it out of the analysis. As stated, this is a problem of: