Depending on the tasks of a particular flood-related project, the selected probability distribution for floods can be used in two ways. One task may be to calculate a *design flood* associated with some level of risk (e.g. probability of exceedence of the design flood), while another task may be to make a *risk assessment* for a specific flood magnitude (e.g. to find the probability of exceeding certain flow rate).

For the task of estimating a design flood, the selected distribution of floods is used to calculate *quantiles* (values of the random variable *X*) for a given probability. In the case of the risk assessment, the selected distribution of floods is used to calculate probabilities for given values of flood variables. Essentially, the procedure of the flood frequency analysis for the two tasks is the same, but the calculations are performed in two opposite directions.