As promise, the second part “Interaction”.
Interaction: When you created your blog, you did it for a reason, an objective, so you should measure if that objective is being achieved or not. Whatever your objective is, it may be related with some kind of interaction between your audience and your blog (unless your objective is not being visited at all…like an autistic blogger ;-)) so, it is important to measure it. Let’s see how to do it:
a. Comments per post: This metric is as simple as useful; because it let you know if your posts are generating interaction among your readers. If you objective are interacting with your readers via comments, then this is your key metric. However if you just receive a few comments (or none) it doesn’t mean that your post was not successful. Some blogs give readers the space to leave comments while others don’t (consciously or unconsciously), so it really depends on you and your blog’s style. If you receive just a few (or none) comments per blog, I recommend asking your readers about it through a poll.
b. Comments per visit: This metric will allows you to find out the level of interaction between your readers and your blog. While comments per posts tell you how “interesting” was a particular post, Comments per visits tells you in average how interested are your users with your content.
c. Internal search per visit: Another way to know how interested your visitors are in your blog is by looking at their internal search behavior. Those users interested in your content will keep looking at other content. However I recommend you to understand the user experience, because some blogs has very smart “content recommenders” which makes people go jumping from content to content without even touching the search box.
Internal searches can provide you another key information, which content is “relevant or interesting” to your readers. Remember that the keywords used to get the site by organic search just tell you how your visitors were able to get your site (depends on if the site was or was not available with a particular keyword) while “internal search” tells you what are they looking for, not matter your site is visible in organic search with that keyword or not.
d. Average time per visit: No matter your blog’s structure (your blog displays the entire post in the home page or not) the Average time per visit helps you to understand if you are getting qualified visitors or not, and if your qualified visitors are interested in your content or not. If your visitors are not qualified you will have the most of your visitors in the segment of less than 30 seconds while the rest of the segments will remain flat or almost flat, however if your visitors are qualified but they are not finding interesting content then even when the less than 30 seconds visits segment will be the highest one, you will also have an interesting quantity of people in the 2 and 3 minutes segments.
e. Average time per page: I just mention this metric to make you be careful about it. The average time per page depends merely on the large and difficulty of the displayed content and not on how interesting or not would it be.
What you can do with this metric is analyzing the top 20 pages with the highest average time per page and try to determine if the one with the highest time has something in common. Once you did it that is the variable you may tune if you are looking to improve that metrics.