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

CodeDisplayDefinition
averageAverageThe [mean](https://en.wikipedia.org/wiki/Arithmetic_mean) of N measurements over the stated period
maximumMaximumThe [maximum](https://en.wikipedia.org/wiki/Maximal_element) value of N measurements over the stated period
minimumMinimumThe [minimum](https://en.wikipedia.org/wiki/Minimal_element) value of N measurements over the stated period
countCountThe [number] of valid measurements over the stated period that contributed to the other statistical outputs
totalcountTotal CountThe total [number] of valid measurements over the stated period, including observations that were ignored because they did not contain valid result values
medianMedianThe [median](https://en.wikipedia.org/wiki/Median) of N measurements over the stated period
std-devStandard DeviationThe [standard deviation](https://en.wikipedia.org/wiki/Standard_deviation) of N measurements over the stated period
sumSumThe [sum](https://en.wikipedia.org/wiki/Summation) of N measurements over the stated period
varianceVarianceThe [variance](https://en.wikipedia.org/wiki/Variance) of N measurements over the stated period
20-percent20th PercentileThe 20th [Percentile](https://en.wikipedia.org/wiki/Percentile) of N measurements over the stated period
80-percent80th PercentileThe 80th [Percentile](https://en.wikipedia.org/wiki/Percentile) of N measurements over the stated period
4-lowerLower QuartileThe lower [Quartile](https://en.wikipedia.org/wiki/Quartile) Boundary of N measurements over the stated period
4-upperUpper QuartileThe upper [Quartile](https://en.wikipedia.org/wiki/Quartile) Boundary of N measurements over the stated period
4-devQuartile DeviationThe 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.
5-11st QuintileThe 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
5-22nd QuintileThe 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
5-33rd QuintileThe 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
5-44th QuintileThe 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
skewSkewSkewness 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)
kurtosisKurtosisKurtosis is a measure of the "tailedness" of the probability distribution of a real-valued random variable. Source: [Wikipedia](https://en.wikipedia.org/wiki/Kurtosis)
regressionRegressionLinear 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.