The reporting may show differences between the percentage of visitors assigned to each variation. For example, you have defined a 50/50 traffic modulation, and your report shows a different split.
These differences may be due to one of the following reasons:
There is not enough traffic on the campaign
When a visitor lands the website for the first time, they are randomly assigned a variation (or the original version). If they visit the website again, they will remain assigned to the same variation.
However, if there is too little traffic on the website, its distribution between the variations cannot be even.
For instance, out of 10 visitors, 7 can be assigned to variation 1 and 3 to variation 2. With 100 visitors, you may have 60 visitors who will see variation 1 and 40 who will see variation 2.
The more visitors visit the website, the more you will tend towards a 50/50 distribution.
We may see a difference of up to 12.4% from a thousand visitors, 4.32% from ten thousand visitors, and 1.1% from one hundred thousand visitors. The difference is only 0.37% from a million visitors.
Traffic allocation was changed after launching the campaign
You can check if traffic allocation was changed via the campaign history tab in your campaign reporting:
If it is the case, it is more likely that traffic modulation won’t be respected because the visitors who had already been assigned to a variation (or to the original version) won’t be impacted by this change. As they already have the information stored in the AB Tasty cookie of their browser, they will always see the same version, meaning that traffic modulation changes will only apply to new visitors.
For example, if your campaign was originally 50/50 and you change it to 70/30, there is a chance your campaign will never actually reach 70/30.
For more information, refer to I deactivated a variation in the traffic allocation step but still see it online.