Statistical Process Control Implementation in Web Analytics: Key Concepts
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Many of you reading this article may not know what “statistical process control” or “SPC” is. It is a problem-detection technique implemented for any measurable process. Keep reading as we delve deeper into the appropriate methods and show how to apply them to SEO.
The word "statistical" implies that the data gathered is being analyzed to determine whether a certain event can be classified as a "common cause" or a "special cause." "Common causes" are random normal noises in any process.
An example of a "process" is a "search engine" with search engine results as an output and its algorithm doing the computations. Random noises always occur in the ranking (e.g minor algorithmic adjustment), which explains the day-to-day fluctuations in ranking, which also results in fluctuations to website traffic.
There is no such thing as a constant ranking in Google or constant daily website traffic. It is impossible to believe your website is getting exactly 105 unique visitors a day for an entire month or that it is ranking at position 9 all through the year. Fluctuations in most common web marketing metrics (unique website traffic or conversion rate) are caused either by so-called "common causes" or "special causes."
On the other hand, a "special cause," as the term suggests, is a rare event that happens in the process that results in a substantial problem. In search engine optimization, a good example of a "special cause" is a "complete overhaul of Google's search ranking algorithm" or a major update, which can cause a substantial change in rankings and traffic.
Another special cause is internal, such as getting your entire website tagged with <meta name="robots" content="noindex, follow"> causing all the indexed pages in Google to disappear, as well as the website traffic associated with it.
Details concerning "common cause" and "special cause" issues are covered in this article, and you will learn whether a certain trend of events in your website can be classified as normal or not. You can also learn about the general definitions of such causes.
Identifying the nature and type of these causes is beneficial to both SEO practitioners and clients. Failing to examine the scenario without clear evidence (such as not using statistical process control) can lead to "over-control" (an excessive reaction to ranking/traffic fluctuations which in fact are normal, like getting position 6 yesterday and position 9 today) and "under-control" (underestimating a problem which in fact is serious, such as having 10 consecutive days of down-trending traffic).
By understanding the concepts of statistical process control, any webmaster, business owner or SEO can properly identify real problems on the spot. This will prevent you from spending a lot of money on situations which are not really a problem at all. A good example is immediately doing on-site tweaks (or purchasing an SEO investigation) by hiring an on-site worker or SEO professional on a ranking that falls from Position 6 to Position 9 or a traffic drop from 1000 to 950. This might sound crazy, but it happens a lot when dealing with paranoid clients and over-reactive SEO.
The objective of this article is to illustrate key concepts of how statistical process control can be used to monitor search engine marketing variables such as web traffic, conversion rate, and so forth to spot special causes (real problems) occurring in the website daily.
Search Optimization 
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