Webtrends Visitor Paradox
Visitor Number Paradox
How can I have more visitors in a week than a month?
Recently we had a client who pulled up two visitor reports. One for the month of December and one for a custom date range, 1st - 25th December. He was startled to find that there had been 25 visitors between 1st - 25th yet only 13 in the whole of December. How could this possibly be true? It simply did not make sense.
To understand what had happened we need to first understand how the Webtrends Analytics package works out how many visitors visit your site in a given time period.
Webtrends will typically have buckets that it lumps visitors into.
If I visit the website today, I will be put into each of the Daily (Monday), Weekly & Monthly buckets.
If I visit tomorrow (and it's the same week and month), I am only put in the daily bucket for Tuesday as I'm already in the Weekly & Monthly ones.
If I visit next week (and it's the same month), I will be put in the daily and weekly bucket as I'm still in the Monthly one.
So, if I want to view a report that shows how many visitors there were for last Tuesday, Webtrends simply looks in the Day bucket for last Tuesday. Similarly if I want to see last week or January it can look in the relevant bucket.
But what happens if I want to view a custom date range? Webtrends does not have buckets for this, so it improvises and creates an approximation by summing visitors from the buckets it does have.
Let's look at an example. We want the custom date range 1st December to 10th December. Webtrends will use a combination of the day buckets and week buckets to give us an approximation.
The problem is that visitors may be in more than one bucket and so could be double counted.
So in our example below, one guy visits on the 1st and then again during Week 1 whereas the rest of the gang only appear in one bucket. If we count up the visitors there are clearly 7 of them. But if we count them by bucket there are 8!
1 4 2 1
And if we look at the month bucket we see there are only 7 visitors.
This is the reason that sometimes, if you look at a custom date range, you will see more visitors in that shorter period than you do in a longer default time period. Because of this double counting you should therefore always be cautious when using custom date ranges and avoid them where possible.
Note that this calculation applies only to visitors. Visits & Page Views can be added together from their different buckets as they are not mutually exclusive between dates!