Dear friends, Asalam o alikum!
Economist and sociologist interested in religious analysis, have to face the problem of measuring religiosity level of individuals. Measuring religiosity is a big issue in the way of researchers from last many years. We do not find any consensus in measuring social capital to date, then a complex matter of measuring religiosity will take huge time to get resolved.
My interest and experience is only related to Islamic side, and I am unable to comment on any other religion. Identifying the indicators of religiosity to develop scales for measuring religiosity is a whirlpool of different theories.
Muslims believe that Allah knows who is more religious and who is less. According to my point of view, Takwa is the standard criteria of religiosity. But Takwa is not measurable because of two reasons; no one know the intention of a person who perform religious prayers, second, Takwa is not only performing obligatory prayers, but it is to stay away from all wrong doings all the time, which are not measurable.
Coming back to the conventional method of counting prayer/mosque attendance (like church attendance), if it is a proxy used as measure of religiosity, it is able to give right results for macro studies with huge sample size. Mosque attendance is a good indicator for religiosity where as it also has some critical issues.
1. Male go for prayers but female don’t go, because it is not obligatory for them in Islamic jurisprudence.
2. Mosque attendance has an alternative of praying at home, in office or in car/vehicle in exceptional cases.
3. It is permissble in Islam to stay at home for prayer when it is raining outside or cold.
4. Shia Islam demands 3 times arrival at mosque for prayer.
5. Exceptions are there in mosque attendence scale also, for example, employee of mosque who works there are Moazzan, Qari have to pray as this is part of their job.
[see the article Issues on measuring religiosity by Hamdani, Syed Nisar Hussain (2007) conference paper, Iran]