 According to KKV, the rules for good causal theories are that they need to be falsifiable, have internal consistency (generate hypotheses that do not contradict each other), have variation (explanatory variables should be exogenous and dependent variables should be endogenous), have "concrete" concepts (concepts should be observable), and have "leverage" (the theory should explain much by little).KKV sees process-tracing and qualitative research as being "unable to yield strong causal inference" due to the fact that qualitative scholars would struggle with determining which of many intervening variables truly links the independent variable with a dependent variable.The primary problem is that qualitative research lacks a sufficient number of observations to properly estimate the effects of an independent variable.