The results of the aforementioned landmark study subsequently were replicated with data from CASS (Coronary Artery Surgery Study)73 and were within 5% of the original estimates for 23 of 24 patient groupings. The single major exception was the category of adults who were #50 years of age with atypical angina, for whom the CASS estimate was 17% higher. On the basis of this high degree of concordance, the data from these studies were merged in the 2002 Chronic Stable Angina guideline7,52,73 (Table 9). Additional validation studies were conducted with data from the Duke Databank for Cardiovascular Disease, which also incorporated electrocardiographic findings (Q waves or ST-T changes) and information about risk factors (smoking, diabetes mellitus, hyperlipidemia).71 Table 10 presents the Duke data for mid-decade patients (35, 45, 55, and 65 years of age). Two probabilities are given. The first is for a low-risk patient with no risk factors and a normal ECG. The second is for a high-risk patient who smokes and has diabetes mellitus and hyperlipidemia but has a normal ECG. A key contribu- tion of the Duke Databank is the value of incorporating data about risk factors into the probability estimate. A limitation of these predictive models, however, is that because they were developed with data from patients referred to university medical centers, they tended to overestimate the likelihood of IHD in patients at lower risk. It is possible to correct this referral (or ascertainment) bias by using the overall prevalence of IHD in the primary-care population,72 although these adjustments are themselves subject to error if the prevalence estimates are flawed. An additional limitation of these models is that they were derived from populations of patients #70 years of age. 