Info about Census Data:

The Census asks different questions each time and they older ones have been very helpful when researching your "roots" as they have what year you immigrated to the US, can you read and write and a lot of other "personal" questions. Look at the questions from the 1910 one and then 2010 and they are WAY different. I feel sorry for when my grandchildren want to go back and research as the information will just not be there.

The hardest part of the newer Census has been what do you mark as your race if you are "mixed?" There is no easy way to say "Well, I'm half Black and Chinese."

Why the Census and other surveys ask about the Hispanic/Latino origins, I have no clue.

Here are some links to what was asked for different census and also what was changed and why:

1910
The 1910 census questionnaire was similar in design to that used in 1900. The most notable change was the late addition, at the behest of Congress, of a question concerning a person's "mother tongue." It was so late, in fact, that questionnaires for the census had already been printed. Information on "mother tongues" was to be added into "nativity" columns 12, 13, and 14.

1930
For the 1930 census, the population questionnaire was basically the same as it had been in 1910 and 1920.The biggest change was in racial classification. Enumerators were instructed to no longer use the "Mulatto" classification. Instead, they were given special instructions for reporting the race of interracial persons.

A person with both White and Black lineage was to be recorded as Black, no matter fraction of that lineage. A person of mixed Black and American Indian lineage was also to be recorded as Black, unless he was considered to be "predominantly" American Indian and accepted as such within the community.
A person with both White and American Indian lineage was to be recorded as an Indian, unless his American Indian lineage was very small and he was accepted as white within the community. In fact, in all situations in which a person had White and some other racial lineage, he was to be reported as that other race. Persons who had minority interracial lineages were to be reported as the race of their father.
For the first and only time, "Mexican" was listed as a race. Enumerators were to record all persons who had been born in Mexico or whose parents had been born in Mexico and who did not fall into another racial category as "Mexican."

1940
The 1940 census was the first to include a statistical sample. Five percent of people were asked an additional 16 questions. In order to gauge the effect of the Great Depression on the nation's housing stock, a census of occupied dwellings was coupled with the usual demographic questions.

The 1940 census was the first to include a separate questionnaire on the condition of the nation's housing stock. Unlike on the general population questionnaire, enumerators were required to check one of a series of options for each question, rather than write in a response.

1960
For the first time in 1960, the Census Bureau mailed out a combined population and housing questionnaire to all urban residents in the United States. Residents were to complete the questionnaire themselves and hold it until an enumerator came visit and collect the form. Enumerators then gave an additional sample questionnaire to 25 percent of households, with instructions to mail it back to their census office. Rural residents were enumerated by traditional visitation. The census "short form" collected only five questions: relationship to head of household, age, sex, race, and marital status

2010
For the 2010 census, the long- and short-form questionnaires used from 1940 to 2000 were replaced by a single questionnaire asking 10 questions. The questions asked by the long-form questionnaire are now asked by the annual American Community Survey


As for WHY they ask the questions they ask...here is a link to a sample 2010 Census Form it is interactive so as you scroll down through the questions a pop-up box explains the reasoning behind the question.