Polit-O'Hara, Denise. 
Nursing research :  principles and methods /  Denise F. Polit, Cheryl Tatano Beck.   - 7th ed.   - Philadelphia ;  London :  Lippincott Williams & Wilkins,  c2004.   - xvii, 758 p. :  ill. (chiefly col.) ;  24 cm.