"The actual Enumeration shall be made within three Years after the first Meeting of the Congress of the United States, and within every subsequent Term of ten Years, in such Manner as they shall by Law direct." (U.S. Const. art. I, § 2)
Normally considered a quantitative, demographic data set, the decennial Census is much more complex. Consider the qualitative aspects of Census data collection. How have the Census questions changed over time? Do different questions for the same data point (e.g. race) change the outcome? Does collectiong data among different populations create challenges, for example, how are incarcerated persons counted?
Here are some other examinations of the Census that go beyond its use as a quatitative data set: