I often use attendance at religious services as a measure of religious commitment (because it is usually the only question available) but there could be many reasons to go church: to please your parents, because you like some girl there, to find business opportunities, etc. How well does the measure actually tap sincere commitment?
Teenagers--individuals who are often forced to go to church--were asked in the Add Health Study: 1) frequency of church attendance, 2) frequency of prayer, and 3) importance of religion to self. I correlated the measures for 6,504 individuals:
Church attendance-Importance .69
Church attendance-Prayer .66
Getting correlations this high from survey data is rare (frankly, it feels good when it happens). Cronbach's alpha--a measure of how well the measures hang together--is .88 which indicates that the measures are tapping an underlying concept--religiosity.
The numbers are this high even though some of the teenage religious involvement is coerced. I conclude that any one of these questions is an adequate measure of religiosity.