In a survey of eating habits, 3800 national adults were selected randomly and contacted by telephone. Respondents were asked, “Typically, how many times per week do you purchase food at a fast-food restaurant or convenience store?” It was found that, on average, those surveyed purchased food at a fast-food restaurant or convenience store 2.4 times per week.

Suppose the researchers that conducted the survey described above are not convinced and decide to make it a long term study. They conducted the same telephone survey many times over a long period of time, each time they would calculate the mean number of times those surveyed purchased food at fast-food restaurant or convenience store. Five years later, after finding the average of over 30 similar telephone surveys, they published a result that states, on average, adults are likely to purchase food at a fast-food restaurant or convenience store 2.8 times per week.

The latest result is believed to be closer to the real number of times an adult would purchase food at a fast-food restaurant or convenience store per week due to