Blocking is not considered. An experiment is performed. An experiment is performed. An observational study is performed with reference to related randomized experiments. An observational study is performed.
Anecdotal evidence is presented.
It is practically impossible or very costly to draw a sample directly from the population. A convenient sample frame exists, but may omit some units of the population. Obtains an SRS from the population claiming it is inappropriate or not useful to draw from a sampling frame unequal to the population, but they note practical concerns. Obtains an SRS from the population of interest with no regard to the practical concerns. Obtains a non-random sample noting practical concerns.