Recognizing that causation is more than correlation highlights a
problem. While we can observe correlation we cannot observe cause. We
have to infer cause. These inferences however are `necessarily fallible . . .
[they] are only indirectly linked to observable' (Cook and Campbell,
1979: 10). Because our inferences are fallible we must minimize the
chances of incorrectly saying that a relationship is causal when in fact it
is not. One of the fundamental purposes of research design in explanatory
research is to avoid invalid inferences.