which type of experiments separates subjects into groups that are similar, but differ in ways that might affect the outcome of an experiment?

Randomized Block design

Explanation:

it is considered to be experimental design in which given units are divided into blocks. For each unit, treatment is allocated inside every block.

In Randomized block design, there is no restriction on a number of times the same treatment is allocated to block. one advantage of Randomized Block design over a completely randomized design is that missing ploy can easily be identified.