The guidance as per the following publications may further help:
Creswell, J. W. (2009) Research Design: Qualitative, Quantitative, and Mixed Methods Approaches. Third edn. London: SAGE Publications Inc. (See : Chapter Six - The Purpose Statement).
Jacobs, R. L. (2011) Developing A Research Problem and Purpose Statement, in Rocco, T.S., Hatcher, T. and Creswell, J.W. (eds.) The Handbook of Scholarly Writing and Publishing. San Francisco, CA: Jossey-Bass, pp. 125-141.
Jacobs, R. L. (2013) Developing a research problem and purpose statement: A guide for doctoral students in human resource development and adult education, New Horizons in Adult Education and Human Resource Development, 25, 3, pp. 103-117.
Pemberton, C. L. A. (2012) A “How-to” Guide for the Education Thesis/Dissertation Process, Kappa Delta Pi Record, 48, 2, pp. 82-86.
Roberts, C. M. (2010) The Dissertation Journey: A Practical and Comprehensive Guide to Planning, Writing, and Defending Your Dissertation. Thousand Oaks, California: Corwin. (See : Purpose Statement, pp.134-135) .