There's not too much to say about formal probability theory. Just three things. 1 if E is a sure thing. The probability that something is true for in an individual selected at random from a population is equal to the fraction of the population for whom it is true. While there's not too much to say about formal probability theory, there's lots to say about probability.
Probability is what makes statistical methods work.
Just as we can construct histograms of samples, we can construct histograms of populations. The idea is the same. The population histogram describes the proportion of the population that lies between various limits, just as the sample histogram describes the sample. However, in addition, the histogram of the population describes the behavior of individual observations drawn at random from the population, that is, it gives the probability that an individual selected at random from the population will lie between specified limits. Instead, we refer to probability densities and distribution functions .