Sandbox
Census errors
Can you trust Census data?
by Justin Wolfers, New York Times, Freakonomics blog, 2 February 2010
Census Bureau obscured personal data—Too well, some say
by Carl Bialik, Numbers Guy column, Wall Street Journal, 6 February 2010
These stories describe problems with the Census Bureau' IPUMS (Integrated Public Use Mircodata Series) data, which provides subsamples of Census data to outside researchers. In order to protect the privacy of citizens, the records are altered slightly. For example, incomes may be rounded and ages may be tweaked by a small amount. Ideally this would make it impossible to identify any particular individual, while at the same time not introducing any important distortion into the overall demographic profile.
Unfortunately, it appears that serious distortions have resulted. A recent NBER working paper details the problems, which seem to be especially pronounced in data for ages 65 above. The Freakonomics post reproduces the following graph from the paper
showing how total population estimates based on the microdata diverge from the actual Census counts for older Americans. Breakdowns within particular age groups are also distorted. For example, The Wall Street Journal article has an interactive graphic, illustrating how data released in 2006 showed inexplicable fluctuations from one age year to the next in the percentage of women who were married (those errors were corrected in 2007).
As Bialik notes, "The anomalies highlight how vulnerable research is to potential problems with underlying numbers supplied by other sources, even when the source is the government. And they illustrate how tricky it can be to balance privacy with accuracy."
Submitted by Bill Peterson