A problem with a causative aspect corresponds to one where interest lies in investigating the nature of a causative relationship between an explanatory variate and a response variate. The preceding language allows us to be more precise about what is meant by `causative relationship'. By this we mean that a change in the value of the explanatory variate (while holding all other explanatory variates fixed) for all units in the population results in a change in the value of an attribute of interest.
A problem has a predictive aspect if the object is to predict the values of variates on one or more units in the target population. A problem has a descriptive aspect if the object is to estimate or describe one or more attributes of the population.
The problem aspect here is descriptive; the aim is to estimate a population attribute, the average speed of light. Had Michelson been attempting to show that the speed of light can be changed by, for example, having the destination move with respect to the source, then the problem has a causative aspect.53 Michelson's work does not easily lend itself to illustrating a predictive aspect. A more familiar example is forecasting future sales from past information.
It is important to decide the aspect at the problem stage because of the special requirements it can impose on the plan.