The differences in the interpretation of perceived findings are one of the most important elements in breast cancer detection. Several studies have shown that radiologists do not necessarily agree with each other, which is reflected in wide ranges of sensitivity and specificity, when groups of radiologists read the same mammogram cases. This variability, however, is not well understood. The characteristics of the areas where the observers agree or do not agree have not been widely explored. In this paper we compare the agreement rates of observers belonging to two different groups, namely, mammographers and residents, when reading a test set of mammograms. We determine the spatial frequency characteristics of areas that yield high agreement, as well as that of areas that yield high disagreement, among the observers.