Pathology Research International 3 Table 1: Summary of results. Consultant pathologists 82 43 10 29.7 0–100 37.1 38 28 10 28 0.008–99 35.6 Trainee pathologists Other doctors Medical students 23 16 10 28.8 1–90 28 59 10 32.3 34.9 0.08–90.1 Number responding to survey Number answering question Median predicted probability of tumour B (%) Mean predicted probability of tumour B (%) Range (%) Standard deviation 25 s t edn 20 n sop 15 e r fo 10 r e bm 5 u N 0 0–9.9 10–19.9 20–29.9 30–39.9 40–49.9 50–59.9 60–69.9 70–79.9 80–89.9 90–100 Predicted probability (%) Figure 1: Distribution of predicted probability of tumour B—consultant cellular pathologists. cellular pathologist group and up to 99% in the trainee group. There were no statistically signiﬁcant diﬀerences between the mean estimate of probability between the four groups, as assessed with either independent two-tailed t-tests or one- way ANOVA assessments (signiﬁcance deﬁned as P < .05). Despite the wide spread of estimates within each group, all four groups produced an identical median estimate probability of 10%. It is of interest that the median estimate of these groups is close to the Bayesian result, whereas the mean results are approximately four times higher. There is no evidence of correlation between experience in making diagnoses (consultants compared with trainees and medical students) and the accuracy of their estimates. The distribution of responses within the groups was distinctly non-Gaussian. Figure 1 shows the distribution of responses within the group of consultant pathologists. There appears to be a bimodal distribution, with most of the respondents towards the left of the chart (estimated proba- bility of 0–29.9%), but with a second peak in the 90–100% category. The distribution also failed to follow a Gaussian pattern in the other groups (data not shown). The question itself, whether due to the presentation or the content, appeared to be oﬀ-putting to some respondents, and it is acknowledged that the explicit description of the probability of a diagnosis is rare in routine diagnostic practice. Only 52% of the consultant pathologists, 74% of the trainee pathologists, and 70% of the nonpathologist medical doctors who undertook the survey provided an answer for the particular question described in this report. 4. Conclusion Despite the limitations of this study, the data would suggest that base-rate error is a potential source of diagnostic error for cellular pathologists. When deciding whether to order and how to interpret the results of an immunocytochemical test, it is very rare to have clear data on the pretest probabilities of a disease in the relevant population and the rate and pattern of positivity in the diseases relevant to the diﬀerential diagnosis. The published literature is often contradictory regarding the rates of positivity for stains in diﬀerent tumours, with subjective factors (including personal thresholds for the interpretation of positivity) and local factors (e.g., variations in ﬁxation, pre-treatment, or antibody clones used) making extrapolation from published literature even more challenging. Inevitably, the interpreta- tion of the diagnostic signiﬁcance of the test results depends on the subjective impression of the pathologist in the context of the clinical information and morphological ﬁndings. The data presented here demonstrate that, at least in the admittedly artiﬁcial setting of a text-based case scenario, con- sultant and trainee pathologists show enormous variation in their interpretation of the diagnostic signiﬁcance of an unex- pected immunohistochemical result. I am using the term “diagnostic signiﬁcance” in this context to refer to the weight placed by the pathologist on the ﬁnding in the determination of their diagnosis and “unexpected result” in this context to refer to a test result that is positive in a higher proportion of cases of an alternative diagnosis than in the diagnosis considered most likely before the test was performed.