So here is my first stab at this - I've calculated this against the actuals of a few communities that are wildly different in size but would love to hear from others whether is works for them as well.
- Total members for a given month
- Total active members for a month
- Total posts (this can be a blog post, a wiki post, a discussion item, a link) for a month
- Total addressable market (how many members would you have if everyone was in the community - this will be a rough estimate)
- ((% of active members * # posts/per member/period) + total members ) / TAM
By including both active members and posts/per member/period you can account for everyone that participates but also your very active contributors since it is important that more people contribute but equally important that new content (no matter who contributes it) is flowing into the community.
By putting the total addressable market in the denominator and total current members in the numerator the metric normalizes across different size communities but it also gives the community increasing credit as it grows closer to the TAM.
Results for me tended to start at 0.001 - 0.01 in the early stages of a community and reach the 0.3 - 0.4 area for very robust communities.
Thoughts? What am I missing or calculating incorrectly?