1. Berger , Carl F. A Piagetian Task Considering the Double Variables of Mass and Volume by Preservice and Inservice Elementary School Science Teachers. April 1976
2. Piagetian Tasks Clarified: The Use of Metal Cylinders;Blake;The American Biology Teacher,April 1974
3. Teaching 'Density'-A Real Problem;Gennaro;School Science and Mathematics,June, 1966
4. The Teaching of Density;Gennaro;Minnesota Science Teachers' Association Newsletter,1978
5. Lawson , Anton Classroom Test of Formal Operations: Testing and Scoring Procedures and Answer Key 1977