An experiment with a three factorial design is described which tests the impact of 1) the degree of the mapping isomorphism, 2) the differences in the types of reasoning (deduction, induction, and analogy), and 3) the Icind of entities changed (objects, attributes, and relations) on the certainty of the inferences made. All the three factors have been found to have significant main effects and a significant interactions between the first factor and all the rest have also been found. Different particular results are discussed. For example, the certainty in the deductive inferences is not significantly different from the one in induction and analogy when there is no one-to-one mapping between the descriptions. Moreover, deduction, induction and analogy have similar behavior in relation to that factor. This is considered as a possible support of the existence of a uniform computational mechanism for evaluation of inferences in all the three kinds of reasoning, a mechanism which is primarily based on the degree of isomorphism.