Central in this study is the question why subjects who possess the necessary factual or propositional knowledge needed to solve a particular statistical problem often fail to find the solution to that problem. Ten undergraduate psychology students were trained so as to possess all the relevant knowledge needed to solve five multiple choice problems on descriptive regression analysis. They were asked to think aloud while attempting to solve the problems. Analysis of the think-aloud protocols showed that a failure to select the relevant information in the text, together with a failure to retrieve relevant propositional knowledge from memory and the inability to reason coherently combined to produce incorrect responses. Factual knowledge was less likely to be successfully retrieved when it was acquired only recently or when it concerned relationships of a highly abstract nature.