Example: Suppose in a class, there are 100 students out of whom 90 are boys and 10 are girls. Then the probability of a student chosen at random to be a girl is
No. of girls in the class
————————————-= 10/100 = 0.1
Total no. of students in the class
Probability can take any value between 0 and 1
Where 1 = certainty
Where 0 = impossible
In the above example, probability of selecting a boy =
No. of boys in the class
————————- = 0.9
Total students in the class
= (1- probability of selecting a girl)
As total probability of selecting a boy and a girl is 1 and since there are boys and girls in the class, the probability of selecting one of them is 1- probability of selecting the other.
In the criminal justice context, the accused is actually factually guilty or factually innocent: there is no third option. Hence p (Guilty, G)+ p (Innocent, I) = 1. Applying ordinary rules of number, this further implies that p(G) = 1- p(I); and p(I)= 1-p(G).
