Discrepancies between WebTrends and Google AdWords

Face it, they are never going to agree.   But that doesn’t mean the differences are random or stupid.

From time to time I compare my data and Google AdWords’ reports, day by day and keyword by keyword.  Sometimes I do it with log data.  I’ve gotta be sure I can have a clear conscience when I tell users that everybody’s doing the best they can.

From this work and things that others have talked about, I have tried to assemble a complete list of reasons for discrepancies. 

In all, I’ve seen differences in any month that range from -10% (WT lower) to +5% (WT higher).  Sometimes it’s spot-on, and it’s usually within 2% or so. 

Sometimes the discrepancies build up gradually, apparently involving long-term slow changes in visitor habits.  #1 below is an example.  Some are relatively sudden, due to changes by another site, by the PPC programs themselves, or by spiders and bots that come into existence and disappear quickly.

In a typical month, for the sites I watch, the effects of the factors I describe here tend to be modest and generally cancel each other out, except for #1 which is fairly constant and usually dominates the other statistical differences.  But over time it’s inevitable that we’ll have occasions when The Perfect Storm happens and the differences get really big.  I’ve even seen instances when the engines report a downturn and we see an uptick.  

Reasons for discrepancies (with Google AdWords or any of the PPC programs) include:

Related to WebTrends settings

1. WebTrends reports on visits while the search engines report on clicks.  Or rather MY WebTrends reports on visits.  I set the paid-term dimension to record only the keywords seen on the first hit of a visit.  I’ve found that a surprising number of visitors do additional searches during their visit or they back out of the site then return by clicking on the same PPC link.  When they do that, the end result is a visit with two (or many) PPC clicks in it – one to start the visit, and others happening in the middle of the visit.  WebTrends (as I set it up) will report one visit while the search engines will report several clicks.  Result:  WebTrends is lower.
2. WebTrends is filtering out some of the PPC visitors, such as your own IP address range.  Result:  WebTrends is lower. 

Invalid clicks

1. Some fraudulent clicking software produces “hits” for search engines’ stats but the bots never actually reach the site.  Later on, the search engine may detect them and  give us a refund.  However, the search engines don’t retroactively change the click reports.  Result:  WebTrends is lower.  (Did you know that Google’s reporting allows you to get a count of the clicks they consider to be fraudulent, by keyword?)
2. Some fraudulent clicking is caught immediately by the search engine and is removed from both billing and click reporting.  If those clicks do reach the site, the clicks still are seen as visits by WebTrends, which does not know they should be removed.  Result:  WebTrends is higher.
3. Prefetch bots (notably the AVG Linkscanner bot we have been talking about for the last few weeks) can follow PPC links and produce false visits in WebTrends, although they may be caught and removed from reporting by the search engines.  We know that our detection methods for this bot are not as good as those used by the search engines.  Result:  WebTrends is higher.

Technical glitches

1. Sometimes the search engines under-report for a day or so.  The cause is usually technical difficulties at their end.  Result:  WebTrends is higher.  These discrepancies are occasional but can be large, often clustering during a two or three week period when search engines make major changes in their systems.  This can happen less than once a year but can be as much as 3% in a month.
2. The search engines sometimes drop our tracking parameters from the URLs, usually because of misprocessing of one of our uploads or a flaw in our uploads coming from our end.  This gets corrected when the search engines notice or we do.  Result:  WebTrends is lower.  It happens rarely, once or twice a year.  Discrepancy varies.

Clicks that don’t get into SDC logs (server logs are not affected)

1. A class of browser add-ins often called “web beacon blockers” will prevent a visitor from being tracked by SDC and other javascript, but won’t prevent that visitor from being counted by the PPC engine if they click on a sponsored search link.  One of the most popular is the Adblock Firefox plugin (there aren’t good numbers on how extensively it’s used).  Result:  WebTrends is lower.
2. If your landing page is longish or slow, and the SDC tag is properly at the end of the page, the visitor may leave before the SDC tag has time to load.  Result:  WebTrends is lower.
3. You have PPC ads directed to a site page (like a special landing page) that you neglected to tag for SDC data collection.  Result:  WebTrends is lower.  For a big campaign, a LOT lower.  Voice of experience here!

Mistakenly tagged links producing real traffic, but not from PPC

1. Search engine spiders can follow PPC ads on other sites and record the entire link in their database, including its PPC tracking parameters.  As a result, they display in their natural search results some links that contain PPC tracking parameters .  If someone clicks on one of those links, the resulting visit is actually a natural search visit but the landing page URL indicates to WebTrends that it is a PPC visit.   In the past, we have seen this to be self-correcting because the search engines eventually notice the discrepancy and change the listed URL to the one they see the most, i.e. the one without tracking parameters.  Results:  WebTrends is higher.  
2. Owners of other sites can click on PPC ads and then add a link on their site using the landing page URL they saw, i.e. one that contains PPC tracking parameters.  The result is a visit that appears to be a PPC visit, but is not.  These tend to be self-correcting over time.  Results: WebTrends is higher.
    

Tags

AdWords, agreement, clicks vs visits, different, Discrepancies, don't agree, PPC

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