 This is because it’s a bit unclear if drift is actually what is being manipulated in Dobzhansky and Pavlovsky’s (1957) experiments, or if it is just population size—and population size does not seem to be a process. Perhaps Reisman and Forber have simply shown that population size is a causal factor of evolutionary change in populations undergoing drift. They also seem to be taking Woodward’s account very literally, in that they seem to think that the “variable” that has to be manipulated must be a variable in a mathematical model; otherwise, one could (at least in principle) manipulate the sampling process by manipulating the population rather than simply the population size; Clatterbuck’s (2015) account, discussed in  section 5,  gives an example of this. (Population size is a variable in mathematical models of drift, whereas sampling, as discussed above, is implicit). One could also conceivably manipulate the environment so that sampling was discriminate rather than indiscriminate or to change the nature of the indiscriminate sampling.