For example, in experiments with animals and sex as a factor, the experimenter has no way to ensure that the animals available are a random sample...

For example, in experiments with animals and sex as a factor, the experimenter has no way to ensure that the animals available are a random sample from all animals of each sex. Typically, for small rodents such a mice, the animals are ordered from a supply company, housed in an animal care facility not under the direct control of the experimenter, and are supplied on an as-needed basis. All than can be done is typically hope for the best. What is the actual population of interest? All animals of that sex? All animals of that sex born in a particular year? All animals of that sex born in that year for that particular supply company?