Kathryn wants to control for intelligence in her study. She…

Kathryn wants to control for intelligence in her study. She has a list of all the participants for her study and their IQ scores. She sorts the list of participants according to their IQ scores and then forms groups, making sure the groups are balanced in terms of IQ scores. Finally, she randomly assigns each group to one of the conditions of her study. Which kind of design is Kathryn using?