Polit, Denise F. 
Essentials of nursing research :  methods, appraisal, and utilization.    - 6th ed. /  Denise F. Polit, Cheryl Tatano Beck.   - Philadelphia, Pa. ;  London :  Lippincott Williams & Wilkins,  c2006.   - xii, 554 p. :  ill. ;  24 cm.