Abstract: After a research on the stochastic property of individual index, the paper examined the individual price index and the individual quantity index of the whole goods of a society under the probability space.On the basis of the examination, the paper made a conclusion that the stochastic vector (X,Y) are approximately subjects to the dual normal distribution, and found the true values of price index EZ and quantity index EY under different economic environment in witch supply or demand plays the major role in different periods of time. The paper also argued that as long as the sample size n of representative goods are getting large, the price index of representative goods will approach the true value of price index of all goods. This shows that the price index of representative goods is a good estimate value of the true values of price index EZ of all goods in a society.