"Risk probability" does not quite make sense, perhaps you mean just how to calculate risk. There are many formulas and methods, a lot of them highly complex mathematical models. Risk calculation is an important subset of portfolio theory.
For the simplest cases, consider some of the following definitions:
* the greatest dive that a stock took over a given historical time period. For example, if stock A dropped 30% maximum over past 5 years before rebounding, and stock B dropped 40% maximum over the same period - then by this metric you can see that stock B is riskier.
* standard deviation of the returns over a historical time period. Take as your data set the prices a stock assumed over the last 5 years daily. You can calculate the standard deviation of this data set. The standard deviation is a measure of risk.
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The answer depends on what you mean by "do". Does it mean calculate individually, calculate the probability of either one or the other (or both), calculate the probability of both, calculate some function of both (for example the sum of two dice being rolled)?
Bayesian probability ; see related link .
Risk
It is the risk.
Comprise risk.