Here is the classic article Did You Really Go to Church this Week? Behind the Poll Data.

Here's a quote:

"Several years ago we teamed up with sociologist Mark Chaves to test the 40 percent figure for church attendance. Our initial study, based on attendance counts in Protestant churches in one Ohio county and Catholic churches in 18 dioceses, indicated a much lower rate of religious participation than the polls report. Instead of 40 percent of Protestants attending church, we found 20 percent. Instead of 50 percent of Catholics attending church, we found 28 percent. In other words, actual church attendance was about half the rate indicated by national public opinion polls."