If you manually change the % split to 100% for Experience B, Visitors who had previously been allocated to A (Control) will stay in their initially-assigned experiences. The traffic change will impact new entrants only.
It is recommended that, if you want to change percentages or greatly affect the flow of people into each experience, you should create or copy a new activity. Otherwise, if you change the percentages on different experiences, it will take a few days for the data to normalize again if many purchasers are return visitors. For example, if your A/B test is split 50/50, and then you change the split to 80/20, for the first few days after that change, the results might look skewed. If the average time to conversion is high, meaning it takes someone several hours or even days to make a purchase, these delayed conversions can affect the reports. So, in that first experience where the number went from 50 to 80, and the average time to conversion is two days, only people from the 50% of the population are converting on the first day of the test, although today 80% of the population is entering the experience. This makes it look like the conversion rate plummeted, but it will normalize again after these 80% of people have the two days to convert.
If you specify the percentage of entrants you want to see each experience. The total for all experiences must equal 100% i.e. If 75 % ( say the number is 100) of Visitors Enters the Activity and you have to be allocated 80% (Control A) / 20% (Treatment B) or any other combination of percentage such that The total for all experiences must equal 100%. As you mentioned above you can have an A/B with A being 15 % and B being 60%
When you want to optimize your activity from the beginning and identify the winning experiences as quickly as possible. By serving high-performing experiences more often the overall activity performance is increased you can use Automated Traffic Allocation
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