Becher, Rhoda. "The effectiveness and efficiency of two training approaches in facilitating the acquisition of conservation of number and / or conservation related skills by 4- and 5- year-old lower socioeconomic status children at various stages of development /." Doctoral dissertation, Ohio State University, 1975. http://rave.ohiolink.edu/etdc/view?acc_num=osu1486986969536201Chicago Manual of Style (17th edition)