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. |
