Some products have siblings or other child ASINs that belong to the parent ASIN. These are used to separate your products by colour or size for example. A/B testing across the entire family of products is now possible with Splitly, but you're only able to A/B test the fields that are identical across all child ASINs.
For example, you sell t-shirts in 3 different colours: blue, red and yellow. The primary image is different to show the t-shirts in different colours. So is the title, as you have appended it with either blue, red or yellow. The rest of the fields are the same: the pricing, the features/bullets and the description are identical across all child ASINs. This means under this situation, you are only able to A/B test the identical fields which are the pricing, features/bullets and the description. Because the primary image and the title are different it is not possible to perform an A/B test.
If you really wanted to test a field that is different across your child ASINs, click the overwrite link when creating a test. But be warned! If you unlock a field, the field will be set to that value for ALL child ASINs. This might not be suitable for your product. You have been warned!
If you don't want to include multiple child ASINs in a test just untick them all and tick the single child ASIN you'd like to test on.