A Comparison of ZIP Code Population Estimates
The census that is conducted every 10 years by the US Census Bureau is the only population estimate that makes an attempt to count every member of the population by sending a survey to every household in the USA. Completion is required. The population size changes relatively little over time. Over a 5 year period, the U.S. population is only expected to grow by 4.1% and an each ZIP code is likely to vary be less than 10%. That leaves an extremely small amount of room for error.
Another way to approximate the population is to use a sampling method. Instead of spending $15 billion to survey the entire US, the Census Bureau spends around $250 million to send the American Community Survey (ACS) to part of the country. The answers (such as the number of people in the household) help to estimate the population as a whole. To get those huge savings, a small percentage of the population is surveyed which leaves a range instead of a precise number. The margin of error that determines the size of the range is larger than the expected population growth.
Another way to approximate the population is to estimate. As confirmed by the Internal Revenue Service (IRS), the number of tax returns filed for a ZIP code can be used to approximate the number of households and the number of exemptions can be used to approximate the population. However, estimates are not perfect.