Pew Research Center often reports on population changes over time in key realms such as marriage and other living arrangements, race and ethnicity, and economic well-being. Generally, we like to report data for several recent decades, although we may focus most closely on more recent years.

Tracking trends can be challenging, however, because data availability does not always keep up with the pace of social change, or because definitions change over time. For example, the U.S. Census Bureau only began reporting counts for same-sex couples and cohabiting couples in 1990 and for people of more than one race in 2000. The option to self-describe as “Hispanic” was not included on all census forms until 1980. In some cases, we can find other sources to provide estimates for earlier years, but otherwise we are limited in what we can report.

As is true of our public opinion surveys, we prefer only to report change when it is statistically significant. We generally will make clear when a finding is not statistically significant.