The study population is the collective of study units for which the values of the variates of interest could possibly be determined. This notion corresponds directly to the frame in sample survey literature. The difference between the attributes of interest in the study population and the corresponding attributes in the target population is called the study error. This is a simple quantitative assessment for numerical attributes but can be challenging to define for graphical ones.
The study units may or may not be part of the target population, as is the case in Michelson's study. Because the distances required to measure the speed of light were so large, it was not practical to have the light travel through even a partial vacuum.54 All of the units in Michelson's study involved the transmission of light through air at a particular location over a specified time period. The source and destination were a fixed distance apart and both remained stationary over the course of the study. Michelson decided to look at transmission of light at one hour before sunset or one hour after sunrise during a few days in June 1879. Within these constraints, he was free to choose the units on which he would determine the speed of light.
The study population and the study units were very different from the target in this instance. Michelson recognized that measuring the speed of light in air would result in a study error. He planned to correct the error by using a factor based on the refractive index of air. Note that this correction is outside the purview of statistical method. It requires contextual knowledge.
The statistical method ensures consideration of the relevance of the study population to the target population by forcing investigators to deal directly with the study error. Criteria beyond the study error such as cost, convenience, and ethics will also be important in determining the study population.