Fraud and errors
Something is different
Errors often tend to be found in situations where something is different.
One technique for the identification of such potential errors is through the examination of patterns, based upon various criteria such as occurrence of round numbers, Benford's Law, distribution of days of week, transactions falling on a Federal holiday, etc.
In each of these cases it is possible to apply a metric to the entire population. Then divide the population into sub-groups and apply the same metric against the subgroup. Then compare the metrics of each subgroup against the population as a whole, using a statistically valid comparison measurement such as Chi-Squared or Kolmogorov-Smirnov (K-S).
A Power Point presentation on the whole process and the background theory is available.