Based on ideas in a post by Avinash Kaushik this plugin calculates the following:

• Raw Author Contribution:
  • average number of posts per month
  • average number of words per post
• Conversation Rate:
  • average number of comments per post without your own comments
  • average number of words used in comments to posts

Download the Blog Metrics plugin

Both for all the time you’ve been blogging, and for the last month, it then adds these values in a page on your WordPress dashboard. This looks something like this: