ATK and Cooks Illustrated are both Christopher Kimball things, so yes, they're more than affiliated. In fact, Cooks magazine now says something about "America's Test Kitchen" on the covers now. (Er, or did, the last time I saw one. We let our subscription go awhile ago but I still see it in the supermarket.)

I like Cooks a lot. Christopher Kimball gets right up my nose, however. The day I remembered to NOT read the editorial in the magazine was a happy day for me.