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Probability discovered by Thomas Bayes in the 18th century.
Probability of a proposition is the chance or likelihood that a proposition is true.
If 1 student in 20 has the flue, the probability is 1 in 20.
Prior:
$P(Sally\ has\ the\ flu) = 0.05$
Suppose there are 5 girls and 15 boys.
Conditional Probabilities
Suppose we learn some more information about the situation.
The probability that Sally has the flu conditional on the patient being a girl is 0.2.
The probability that Sally has the flu conditional on the patient being a boy is 0.
We write this as:
$P(Sally\ has\ the\ flu\ |\ the\ flu\ patient\ is\ a\ girl) = 0.20$
$P(Sally\ has\ the\ flu\ |\ the\ flu\ patient\ is\ a\ boy) = 0$