.<p>odd&;s ratio (cross product ratio)is calculated as the ratio between total no of people exposed who developed disease(a) by people exposed and no disese(b) with that of people who had no exposure ,but developed the disease(c),by people without exposure,without disease(d).here it is 30by 50 divided by 10 by 50 .equals 3</p><p>ref;park&;s textbook of preventive and social medicine,22 nd edition,pg no 70</p>