title: Equivalence formation by retarded and nonretarded children at different mental ages.
mesh_terms: D000367:Age Factors; D002648:Child; D002657:Child Development; D003071:Cognition; D003210:Concept Formation; D006801:Humans; D008607:Intellectual Disability; D007361:Intelligence Tests; D011340:Problem Solving; D013647:Task Performance and Analysis; D014796:Visual Perception