The Empty cells method (Grimson 1993) can detect temporal clustering in single and several time series with group-level data. The empty cells test statistic, E, is a count of the number of time periods (cells in a table) with no cases. This statistic can detect temporal clustering where one or more time periods have several cases while other time periods have none. When cases cluster the test statistic will be large.
Use this method to detect clusters of rare events, when some of the time periods can be reasonably expected to have zero cases.
