3.1. Study on the Sample Variance of Each Layer. Obviously,
the sampling rate of each layer is related to the variance of
variables in the layer. Next, we will discuss the variance for
random sampling for each layer. To use the Mathematical
Statistic Method to study the sampling rate, we should
introduce a random variable; here, we use a relatively simple
and intuitive variable, daily average frequency of trips, as
the random variable. We use capital letter