bohm I have made the argument elsewhere that congregation number is the best measure of growth or decline that there is for JWs (or religions generally) for a number of reasons:
1. Unlike almost every other figure, it cannot easily be faked. Congregation names, locations, and meeting times are publicly listed. If they claimed more congregations than actually exist they would get caught.
2. Definitions of everything change over time: publisher (15 minutes), pioneer (how many hours?), Bible Study (door step study, family study?), making diachronic analysis sometimes difficult. Among all the data that are collected there is a good argument to be made that the definition of a "congregation" is among the more stable.
3. Another way of looking at the issue is that growth or decline involves slightly more than simply number of adherents. There is the component of commitment and enthusiasm to consider. And the number of congregations captures this slightly better than the pure publisher number. Because it takes commitment to form and maintain a congregation and to fulfill all the duties associated with a congregation. If the number of congregations begins to decline before the publisher number declines (as is often the case in various countries) this tells us something important and is indicative of "decline" in a broader sense, I would suggest.
4. The countries that are showing real decline show that decline most severely in the number of congregations. Take Japan for example that has declined by around 3 or 4% in terms of publishers, but by over 20% in terms of number of congregations. That's a huge indication of decline that is somewhat masked by the publisher figure.
5. Plus, and this is one of the best reasons for focussing on congregation number in my view, it makes JW figures easily comparable with other religions. It is easy to get bogged down comparing JWs with other religions such as Mormons or Baptists or whatever, because they have wildly different concepts of: adherents, members, publishers and so on. But the concept of a "congregation" is common to all and remarkably similar. Mormon claims to have more members than JWs in the UK, for example, are belied by the fact that they have fewer than half the number of congregations. Plus over time, no organisation that is truly growing will decrease in terms of number of congregations.
Quite simply congregation number is the best measure of growth across time and in comparison with other religious groups.