Functions
average gets the number expressing the central or typical value in a set of data
CDF return the CDF using the mean and the standard deviation given https://en.wikipedia.org/wiki/Cumulative_distribution_function#Definition
erf returns the error function integrated between zero and x. Error function.
erfc returns the error function integrated between x and infinity called complementary error function
CDF return the CDF for the zscore given https://en.wikipedia.org/wiki/Cumulative_distribution_function#Definition
normal_probability_density return the PDF with z already normalized https://en.wikipedia.org/wiki/Probability_density_function
probability_density normalize x using the mean and the standard deviation and return the PDF https://en.wikipedia.org/wiki/Probability_density_function
std_dev return the standard deviation, the square root of the variance
std_err is the standard error, represnting the standard deviation of its distribution
variance us the mean of the sum of all square deviation
the zscore represente the distance from the mean in stddev