> a data set is not completely worthless if it showed a null result As the other commenters have pointed out, the fact that it showed a null result is not real

> a data set is not completely worthless if it showed a null result As the other commenters have pointed out, the fact that it showed a null result is not really relevant to future utility. A better way of phrasing it might be "A data set is not completely worthless after it has been used to the test the hypotheses for which it was gathered." As to how it is useful, I think a sound summary would be "a data set is _only_ useful for validating (or invalidating) those hypotheses for which it is gathered, but it may be useful for deriving new hypotheses for which new data can be gathered to validate." Or more negatively as "a data set is not useful for validating or invalidating hypotheses invented after the data set is seen, but it may be useful for deriving those hypotheses and leading to further research." To some very limited extent, if a mechanism of action is established (which is to say there is an external reason to infer a correlation) then cross-validation with an existing data set may be able to derive the parameters of that correlation.