The very complicated, and important, issue of comparability of racial data is a special case arising from differences in the way Census questions are answered. Major changes were instituted for the 2000 Census in the collection of racial data; therefore the most pressing problems in comparability occur when 2000 data is compared with that from earlier years. In all cases, modifications in definitions and questionnaire structure from Census to Census may cause problems with comparing data.
Prior to 2000, there were complaints from many different sectors of the population that the Census Bureau's collection of racial data was too restrictive, since respondents had to choose one and only one racial category for their answer. People who identify as multi-racial were forced to choose one or the other race, but not both. With the 2000 Census, respondents were able to mark more than one category from a list of major racial categories: American Indian, Asian, Black, Native Hawaiian, White, and other. Given the possible permutations of this list, there end up being 63 different racial categories for which data is being reported. Since in previous years, there were only 6 (or however many categories were offered as answers to the question) it is obvious that racial data from 2000 gives a much better picture of the diversity of the country. But the data from 2000 cannot be directly compared with that from any earlier Census.
Another major departure for Census 2000 lies in the categories offered as choices for Asian- and Polynesian-Americans. Prior to 2000, there was only an "Asian and Pacific Islanders" category. In 2000 these major categories were broken into two. Looking at the statistics for sub-categories of these groups (for example: Chinese, Japanese, Filipino, Hawaiian, Samoan, etc.) might be a way to find comparisons with earlier data.
The biggest problem in finding and using data on the Hispanic population comes from confusion between the concepts of race and ethnicity. Quoting from Census's Overview of Race and Hispanic Origin, "The federal government considers race and Hispanic origin to be two separate and distinct concepts." Hispanics can be of any race. Unfortunately, reporters and others in the media don't always understand this distinction, and one often sees new items comparing Hispanics with African Americans, for example, adding to the confusion surrounding this issue.
In addition to this fundamental confusion, however, Census collection of data on Hispanics has varied so much over the last 60 years that comparisions are practically impossible between any two Census' data on the topic. Data has ranged from "mother tongue" to a comparison of names with a list of Spanish surnames, to "Spanish origin" to "Spanish ancestry," etc. It is important to note that 1970 was the first Census to ask specifically about Hispanic origin and in that Census it was asked only on the sample quesionnaires.