To quantitate the intrinsic error in measurement, fifty anteroposterior
radiographs of patients who had scoliosis were each measured on six
separate occasions by four orthopaedic surgeons using the Cobb method. For
the first two measurements (Set I), each observer selected the
end-vertebrae of the curve; for the next two measurements (Set II), the
end-vertebrae were pre-selected and constant. The last two measurements
(Set III) were obtained in the same manner as Set II, except that each
examiner used the same protractor rather than the one that he carried with
him. The pooled results of all four observers suggested that the 95 per
cent confidence limit for intraobserver variability was 4.9 degrees for Set
I, 3.8 degrees for Set II, and 2.8 degrees for Set III. The interobserver
variability was 7.2 degrees for Set I and 6.3 degrees for Sets II and III.
The mean angles differed significantly between observers, but the
difference was smaller when the observers used the same protractor.