Most of the time, “target population” and “population” are synonymous. However, adding the word target emphasizes that sometimes we miss the mark in sampling, and don’t always hit the mark: samples can be unrepresentative of the population that you originally intended to sample. For example, you might want to survey all the hospitalized adults in the United States (the target population), but budget constraints limit your survey to hospital patients just four cities in the U.S. The sampled population and targeted population in this scenario are likely to be quite different.
Target Population Units
In some areas like regression analysis in epidemiology, it’s especially important to identify the target population. While data analysis in the sciences always includes the correct units (e.g. was time measured in seconds, decades, or light years?), specific information about the population is left out. For example, let’s say you’re target population is people living in the United States: are you including just citizens? Resident aliens? Refugees? If you run a regression involving people without specifying exactly who the target population is, it leads to “hopeless ambiguity” (Rothman et. al, 2008)
References
Gail, M. & Benichou, J. (2000). Encyclopedia of Epidemiologic Methods. John Wiley and Sons.
Rothman, K. et al. (Eds.) (2008). Modern Epidemiology. Lippincott Williams & Wilkins.