Aggarwal, Y.P(2012). Statistical Methods : Concepts, application and Computation (3rd Rivesed), New Delhi : Studying.
Benson, J., and Clark, F.(1982), A Guide for instrument development and validation. The American Journal of Occupation Therapy, 36(12), 789-800.
Burieta J. Standardization of process, Available on the internet : http://ipaslovakia.sk/sk/ipa-slovnik/standardization-procesov-2007.