It is possible to apply this idea of direct and indirect testing to more complicated experiments in order to explain the presence of a congruence bias in people's reasoning.
When subjects get confirmatory feedback from repeated testing of the same rule, their confidence in their assumption increases.
The actual rule used by the experimenter to generate the example and to assess the test sequences provided by the subject was simply "list ascending numbers".
Subjects failed to identify the rule due to their inability to consider indirect tests of their hypotheses.
Wason attributed this failure of subjects to an inability to consider alternative hypotheses, which is the root of the congruence bias.