Four randomly chosen market research companies each produced population estimates for three middle-sized cities; the estimates of each company were then compared with those of the other companies  Two of the cities had relatively stable populations, and for them estimates of current population and of projected population in five years varied little from company to company
However, for the third city, which was growing rapidly, estimates varied greatly from company to company
The market research companies are likely to be equally reliable in estimating the population of stable cities.