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Susan Ring Andrews, Janet Berstein Blumenthal, Dale L. Johnson, Alfred J. Kahn, Carol J. Ferguson, Thomas M. Lasater, Paul E. Malone, Doris B. Wallace, The Skills of Mothering: A Study of Parent Child Development Centers, Monographs of the Society for Research in Child Development, Vol. 47, No. 6, The Skills of Mothering: A Study of Parent Child Development Centers (1982), pp. 1-83
Andrews et al. (Fri,) studied this question.