In order to know how the products in a campaign are performing, you need to be able to compare them against something. Pulse determines the average of how products were performing before a campaign on a number of different metrics. You can then use the comparison to see how products are performing during the campaign.
The length of time of that is used for comparison dynamically updates based on how long the campaign is running for. Pulse will take the time period the campaign is running for, and then take the averages of the previous instances of that same time period. Shorter campaigns will have more instances for comparison, longer campaigns will have fewer.
In short, a campaign running for a short few days will be compared against multiple smaller time periods than a campaign running for a month, which will be compared against fewer but longer periods of time.
The aim is to get as best a comparison as possible. Pulse will also do an analysis of the entire store for the campaign period, so you will be able to easily see a store's overall trend, and use that to determine if the campaign's performance is in line with the store trend for the same period.
Old Campaign comparison method
In the old way of measuring campaigns, the standard comparison period was 5 weeks. Pulse would take the average performance of products over the previous 5 weeks and have a comparison.
Pulse would also do a comparison against the previous 52 weeks for some campaign types.
While both these measures are reasonable, the new method offers better comparisons to help you understand what is going on in your store.