Comparability and compatibility: issues in combining data from central cancer registries.

Before combining or comparing data from different registries, one should consider similarities and differences in data collection methods, data quality, and underlying populations. What are important population demographic differences? What about differences in data quality: How can these be measured and evaluated? What factors can affect data compatibility? How can one assess data comparability? If registries are compatible, are they always comparable? Are comparable data from registries compatible data? When data are combined, what issues should be considered to determine whether the combined result is meaningful? These are some of the common questions that need to be addressed to determine whether and when data from different registries should be combined or compared.