Under certain conditions, observing or testing the studied object is called a test every time the condition group is realized. The result is called an event. In experiments, events that may or may not occur are called random events.
The probability p(A) of random event A is a measure of the probability of this event. Its value is between 0 and 1. Under certain conditions, if event A is impossible, then p (a) = 0; If event A must occur, then p(A)= 1. With the increase of the number of experiments n, the probability of frequency approaching is greater.
Extended data
The random variable X, which describes hydrological stochastic phenomena, generally belongs to continuous type. Therefore, the probability that x is equal to any number x is p{X=x}. Cumulative percentage curve FX (x) ~ x is used to describe the statistical characteristics of hydrological variables in hydrological calculation. If the probability of annual flood peak discharge at Yichang Station of the Yangtze River is greater than or equal to 80000m3/s, p{X≥80000}=FX(80000).
In hydrological calculation, the frequency density function FX(x) of hydrological variables is generally estimated by statistical analysis based on measured data, and then the cumulative percentage function fX(x) of hydrological variables can be obtained by integrating fX(x):
In hydrological calculation, it is customary to call cumulative percentage curve FX(x) as frequency curve and FX (x) ~ x curve as frequency density distribution curve.
Frequency is not equal to probability. According to Bernoulli's theorem of large numbers, when n tends to infinity, the frequency fn(A) approaches the probability P(A) in a certain sense.
Baidu Encyclopedia-Frequency