Evaluation of the strength of the client's internal audit function by the external auditor (EA) has taken on increased significance due to stronger regulation around the evaluation of internal controls after SOX (2002). However, research examining how this evaluation occurs in practice is mixed and inconclusive. In this study, we examine empirically whether the Desai et al. (2010) theoretical model is reflective of how auditors make judgments about the strength of their client's internal audit function in practice. Specifically, we present external auditors with evidence about internal auditor work performance, competence and objectivity in a manner consistent with the structure of evidence evaluation implied by the Desai et al. (2010) model. We then compare the auditors' actual strength judgments to the strength levels predicted by the model and evaluate similarities and differences. Results indicate that no one factor dominates the strength judgment in all cases. In addition, EAs do not weigh negative evidence as heavily as does the model. When the evidence about the three factors is conflicting, external auditors have difficulty incorporating them in a consistent way into the calculation of their overall strength judgment. Finally, we find results consistent with prior research indicating auditors tend to be more sensitive to negative than positive evidence. Also, it is harder to move auditors' beliefs away from a negative position with positive evidence than to move those beliefs away from a positive position with negative evidence. Results suggest that additional training and use of a decision aid structured according to the Desai et al. (2010) model would be especially useful when evidence about internal auditors' work performance, competence and objectivity is conflicting.