Hi
I'm actually going to be addressing this exact question on a post for Moz in the coming weeks - so keep an eye out for that.
But in short, here's what I do;
Analytics
- run a report for landing tag pages (with a filter) - over the last three months
- apply an advanced segment to see google only traffic
- dump the report into a CSV
Webmaster Tools
- view a impressions / clicks report by top pages (not keyword) - also zoom out as far as you can
- filter for web only (not images)
- dump the report into a csv
VLookup in Excel
using a VLookup in excel - combine the two reports matching data to the URLs (you'll end up discarding some non-tag pages from wmt) - the end result will be a master spreadsheet, with the following columns;
- URL
- Impressions
- clicks
- avg position
- visits
- pages/visit
- avg visit duration
- % new visits
- bounce rate
(These are all the default report metrics. I actually prefer a custom landing page report in analytics, but this works fine.)
Analyze
Then, you do your sorting, filtering etc - to decide how valuable the tag traffic has been. In general, you're trying to look for an overwhelming reason for the value add of having those pages in there. they might get visits, but what's onsite behavior? maybe they get visits, but perhaps only from a small handle of tag pages?
In the post I do, I'll cover more about how to analyze this report etc.
As Klarke put so well, the actual posts should rank in their place. Those tend to have better results when people land on those.
Remove
If you decide to remove, do so carefully. Do it on a weekend or just before a downtime. If you use Yoast simply select to noindex tag archives.
Also, rememeber to exclude tags from your XML sitemap.
Then watch webmaster tools etc and watch for their removal.
--- I did this process on a site with 9,000 tag pages in the index and results were very good.
-Dan