1. 1. Rothman KJ, Greenland A, Lash TL: Modern Epidemiology, 3rd ed., 2008; Lippincott Williams & Wilkins, Philadelphia.
2. 2. Hulley SB, Cummings SR, Browner WS, Grady DG, Newman TB: Designing Clinical Research, 3rd ed., 2007; Lippincott Williams & Wilkins, Philadelphia.