Tumors accumulate large numbers of mutations and other chromosomal abnormalities, but not all of these abnormalities are crucial for the tumor.  One important indicator of the role of the abnormality is the order in which it occurred, relative to other abnormalities. We cannot directly observe the temporal ordering of genomic abnormalities, but we have previously proposed a probabilistic model to estimate the relative time of occurrence, based on the allele frequencies of mutations within regions of copy number aberration. This method gives insight into the progression of the tumor, but remains only a fragmentary picture for a single tumor sample. We propose methods to analyze the orderings in order to find common temporal characteristics across a larger collection of tumors.