Thirdly, with a survey you can spend your time and money making sure that you collect information on all members of your sample. You can revisit houses where you didn’t find people at home the first time. This is important because certain parts of the population are harder to reach than others. For example, women, older people, and unemployed people are all more likely to be at home when an interviewer visits. These demographic groups may have different answers to survey questions than their counterparts, and a sample that over-represents them may be biased. CRRC interviewers randomly select a respondent in each selected household. If that household member isn’t home, the interviewer schedules a re-visit to the household, and makes a total of three visits to attempt to find that household member at home. This ensures that the sample contains a representative mix of men and women, young and old, employed and unemployed.
The reasons listed above are all interrelated – time, money, and manpower are always limited, and conducting a survey allows an organization to gain as much information as possible for the resources that they expend. However, in some cases the situation is even more extreme – in some cases, the object of measurement has to be destroyed in order to be measured. Think of how a manufacturer measures the number of calories per cookie: they burn a cookie in a machine called a bomb calorimeter, shown in the figure above. The number of calories in the cookie is a measure of how much heat the cookie produces when burned. Not every cookie is identical, so manufacturers take a sample of cookies. They burn each one in a bomb calorimeter, and report the average number of calories generated per cookie in the sample. If they performed a census on the population of cookies and burned every cookie, there would be nothing left to sell.
