This code system http://terminology.hl7.org/CodeSystem/observation-statistics defines the following codes:

Code	Display	Definition
average	Average	The [mean](https://en.wikipedia.org/wiki/Arithmetic_mean) of N measurements over the stated period.
maximum	Maximum	The [maximum](https://en.wikipedia.org/wiki/Maximal_element) value of N measurements over the stated period.
minimum	Minimum	The [minimum](https://en.wikipedia.org/wiki/Minimal_element) value of N measurements over the stated period.
count	Count	The [number] of valid measurements over the stated period that contributed to the other statistical outputs.
total-count	Total Count	The total [number] of valid measurements over the stated period, including observations that were ignored because they did not contain valid result values.
median	Median	The [median](https://en.wikipedia.org/wiki/Median) of N measurements over the stated period.
std-dev	Standard Deviation	The [standard deviation](https://en.wikipedia.org/wiki/Standard_deviation) of N measurements over the stated period.
sum	Sum	The [sum](https://en.wikipedia.org/wiki/Summation) of N measurements over the stated period.
variance	Variance	The [variance](https://en.wikipedia.org/wiki/Variance) of N measurements over the stated period.
20-percent	20th Percentile	The 20th [Percentile](https://en.wikipedia.org/wiki/Percentile) of N measurements over the stated period.
80-percent	80th Percentile	The 80th [Percentile](https://en.wikipedia.org/wiki/Percentile) of N measurements over the stated period.
4-lower	Lower Quartile	The lower [Quartile](https://en.wikipedia.org/wiki/Quartile) Boundary of N measurements over the stated period.
4-upper	Upper Quartile	The upper [Quartile](https://en.wikipedia.org/wiki/Quartile) Boundary of N measurements over the stated period.
4-dev	Quartile Deviation	The difference between the upper and lower [Quartiles](https://en.wikipedia.org/wiki/Quartile) is called the Interquartile range. (IQR = Q3-Q1) Quartile deviation or Semi-interquartile range is one-half the difference between the first and the third quartiles.
5-1	1st Quintile	The lowest of four values that divide the N measurements into a frequency distribution of five classes with each containing one fifth of the total population.
5-2	2nd Quintile	The second of four values that divide the N measurements into a frequency distribution of five classes with each containing one fifth of the total population.
5-3	3rd Quintile	The third of four values that divide the N measurements into a frequency distribution of five classes with each containing one fifth of the total population.
5-4	4th Quintile	The fourth of four values that divide the N measurements into a frequency distribution of five classes with each containing one fifth of the total population.
skew	Skew	Skewness is a measure of the asymmetry of the probability distribution of a real-valued random variable about its mean. The skewness value can be positive or negative, or even undefined. Source: [Wikipedia](https://en.wikipedia.org/wiki/Skewness).
kurtosis	Kurtosis	Kurtosis is a measure of the "tailedness" of the probability distribution of a real-valued random variable. Source: [Wikipedia](https://en.wikipedia.org/wiki/Kurtosis).
regression	Regression	Linear regression is an approach for modeling two-dimensional sample points with one independent variable and one dependent variable (conventionally, the x and y coordinates in a Cartesian coordinate system) and finds a linear function (a non-vertical straight line) that, as accurately as possible, predicts the dependent variable values as a function of the independent variables. Source: [Wikipedia](https://en.wikipedia.org/wiki/Simple_linear_regression) This Statistic code will return both a gradient and an intercept value.