The answer for this is "D. Bayley evaluates babies by manipulating them to see what they do; Pikler observes what they do on their own."
Nancy Bayley used a lot of measures and scales of infant development, one task of which is to record the observations made of how a child responds to the task that is presented. Emmi Pikler however, wanted the child to learn naturally if possible, without the child being stimulated to a movement or environment.