The population estimates for some metro areas are not always agreed upon. In some cases, different sources give numbers of people which differ by millions. The definitions used for the last U.S. Census differed from those for previous censuses. This makes it hard to compare official information from different dates. MSA boundaries do not stretch into Canada or Mexico. This can affect the number of people in several cities. For example, Detroit, Buffalo, El Paso and San Diego are often much larger than their MSA figures.