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Behind the Numbers: A Tale of Two Census Surveys

When it comes to CPS versus ACS data, this is how we often feel.

Each year around this time, the U.S. Census Bureau releases new numbers from the previous year on things like the number of people who lack health insurance and the number of people living in poverty.  Here at Voices for Children, we look forward to having new numbers that tell us how our state’s kids are faring and whether or not things have gotten better or worse.  But as with all things data, there’s more to it than just simply taking the numbers at face value.

That’s because the Census Bureau actually releases the results of TWO different surveys that track some of the same trends.  Today, the numbers are from a survey known as the Current Population Survey (or CPS, for those is the data biz).  Next week, we will see numbers from the American Community Survey, or ACS.

Sometimes the difference between numbers in these two surveys is not statistically significant, and other times, the numbers are quite different.  For example, in 2010 the CPS reported that there were about 47K uninsured kids in Nebraska, while the ACS reported that number at about 25K.  Why?  There are significant differences in the methodology of the two surveys.  For one, the two surveys are asking different questions.  The ACS asks whether someone has health insurance coverage at the time of the survey, while the CPS asks whether someone has had health insurance coverage at any time during the past 12 months.  In addition, the CPS is actually conducted by an interviewer, while the ACS is done via mail.

There’s also another very important difference between the two surveys that may bring you back to that stats or research class you were forced to take in college — sample size.  For those who are a little rusty on their research methods, sample size refers to the number of people surveyed.  The ACS boasts a sample of about 3 million households while the CPS only surveys about 100K households.

This difference is why we here at Voices are waiting with baited breath for the ACS data next week.   For state level data, the larger sample size gives us a better picture of how Nebraska kids are doing by virtue of having surveyed more Nebraska households.  In fact, the Census Bureau recommends that we look to the CPS for national data, but use the ACS for state information.

So, as the new numbers from the CPS are released today, we will take a minute to check in on how we’re doing as a country, but we’ll be waiting for the ACS data next week to asses the situation for Nebraska kids.



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