I know that many website owners often focus on SERP rank as a measurement of success, but I don't feel like rank is a good metric because of how personalized the search results are getting. There are too many variable and rankings change too frequently to be a true measure of success on their own. I've always preferred to look at visitor growth as one of the main ways to measure SEO success. I should point out, that a site that ranks well tends to drive more traffic, so the two metrics help create a clearer picture. However, there are a lot of components that contribute to visitor growth (or loss) that make it dangerous to view your SEO successes and failures in a silo.
Here are 4 factors contributing to SEO success and visitor growth:
1. PPC Campaigns
Did you know that Google Analytics can break down your traffic into paid and nonpaid? Lets say your paid traffic account for 20% (200) of your total visitors (1000) in a month. If you were to cut back on your PPC spending (lets say you halved it one month due to budget issues) don't be surprised if your paid traffic takes a hit. If you lost 30% of your paid traffic that's 60 fewer visitors from your total traffic. Does this mean that your SEO campaign is failing? Not at all.
2. Branded searches
I had a client that lost a decent amount of traffic one month, nothing to be worried about but more than they were expecting, so I started digging through their analytics. All their major keywords were still driving about the same amount of visitors from one month to the next, but their branded searches (specifically the company name) were way down. Maybe they pulled some advertising campaigns, maybe they had a strong PR push in the previous month that drove branded traffic that wasn't being enforced anymore"whatever the reason, the loss of branded traffic was impacting their overall visitor growth, but their organic SEO was holding strong.
Another client of mine recently launched a microsite. Normally I don't support the idea of microsites for SEO, but the purpose of this site (which has a great URL) is to create and publish thought leadership content for the industry in general, not just help my client rank for certain keywords. Their brand is actually minimally represented on this microsite as they want it to stand on its own merit and build a brand for itself. The new site launched a few months ago, but while the site was under construction the URL was redirecting back to my clients main site, sending several thousand visitors a month. Now that the new website is up and running, my clients site took a pretty noticeable hit in traffic since they aren't getting that redirected traffic.
4. Seasonality and Search Volume
Every business is subject to seasonality, even if they don't line up with the actual seasons. There are going to be fluctuations in search volume throughout the year that impact how much traffic is being sent to your site. For instance, search volume for lawn moving service skyrockets in May, but is virtually nonexistent in November and December. A lawn moving or landscaping company is going to see a decrease in visitors in the winter simply because there are less people searching. This doesn't mean their SEO campaign is no longer working, there just isn't any traffic to be had.
There are a lot of other factors that can impact overall visitor growth or loss for a website, which is why you can never put your SEO in a silo. If you see something that looks out of the ordinary (for good or bad) do a little investigating and see if you can determine what the underlying cause it. Google Analytics lets you make notes in your account, which is a great way to keep track of any major PR or advertising campaigns that lead to a jump in traffic, or any site issues that may have resulted in a decrease. That way you'll always know what happened to your traffic.