The question we are asked most often by the press and the public, “How many uninsured people are there in Massachusetts?”

It’s a simple question, but there’s no simple answer.

Counting the uninsured is tricky. This CBO study goes into some of the difficulties – do you mean uninsured all year, part of the year, or at a specific point in time? Survey methodologies make a difference – for example, a phone survey misses some very poor people without phones.

Today and yesterday we got an example of the complexity.

Today the Census Bureau released its annual study of Income, Poverty and Health Insurance Coverage, for 2005. The Massachusetts Budget and Policy Center summarized the health insurance findings for Massachusetts, as well as the poverty and income data which are also included (headline: “although 2005 marked the fourth full year of growth in the national economy since the end of the 2001 recession, poverty in Massachusetts did not significantly decline and median household incomes did not make any real gains”).

Looking at the raw data for Massachusetts, the percent uninsured fell from 11.7% in 2004, to 9.8% in 2005. The numbers dropped from 745,000 to 618,000.

This drop is parallel to the numbers rushed out yesterday by the Commonwealth, which showed a drop from 460,000 uninsured in 2004 to 372,000 in 2006. The Division of Health Care Finance and Policy estimated the 2006 percentage of uninsured at 6%. EOHHS Secretary Tim Murphy issued a statement noted the similar downward trends between the state and federal numbers, saying it was “very encouraging as we move deeper into the important implementation phase of healthcare reform.”

We agree, but . . .: The Census urges us not to look at the raw numbers. Because of the small sample sizes, the Bureau’s statisticians maintain that to be accurate, one must average at least two years of data. Also, one must take into account the “confidence interval” – the range with a 90% likelihood that the results are accurate. That’s how they present the numbers in their report.

Viewed this way, the Massachusetts uninsured rate as determined by the Census Bureau went from 11.2% for 2003-2004 to 10.7% in 2004-2005, with a confidence interval of 0.7%. Their bottom line: no statistically significant change in the uninsured rate in Massachusetts. In the Census map of states’ uninsurance rates (p. 26 of the report), Massachusetts doesn’t show an increase or a decrease, but simply no significant change.

So, how many uninsured are there? Probably more than 372,000, and certainly less than 618,000. Last year, Blue Cross Blue Shield Foundation’s Roadmap to Coverage project issued a report on the uninsured that used state figures to adjust the census numbers to come up with a consensus number (532,000 uninsured) that was used by the legislature and the administration. That’s probably the most accurate count we can get.