I mostly come from a scientific rather than a philosophical perspective, but here goes.
Frame challenge: I feel like this question is backwards. One doesn't propose a theory and then ask, "What data could this apply to?" It's the other way around: You start with observations (data), then proposed a hypothesis to explain it.
This is hard to see, for example, with Russell's five-minute hypothesis. But only because his starting point is so broad. He is starting with all human experience. (And then proposing—for the sake of argument—the five-minute hypothesis as an idea to be disproven.)
Taking the Russell example further: Theories are made to explain. If some data can be explained by a hypothesis, it's relevant. The other side of the coin is that if some data could disprove a hypothesis, it's relevant.