 While true experiments follow this procedure, other types of studies allow non-random assignment to conditions. Astronomers, for example, sometimes speak of natural experiments. Although we are in no position to affect the motions of heavenly bodies, we can observe them under a variety of conditions, over long periods of time, and with a variety of instruments. Likewise, anthropological investigation of different cultures’ moral beliefs and practices is unlikely to involve manipulating variables in the lives of the members of the culture, but such studies are valuable and empirically respectable. Still, inferring causation is more difficult with such studies, so the evidential value of these studies is often less than that of experiments, at least in this respect; most published research in experimental philosophy involves true experiments.