The primary variate of interest, which we call the response
variate, is the speed of the light associated with each such transmission.
There are
many other variates, which we call explanatory variates
attached to each unit such as the distance between the two
points, the motion of the points with respect to each other, properties of the
source and so on.
In Michelson's problem, he has no direct interest in these other variates.