# Functions for descriptive statistics

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Descriptive statistics aim at characterising empirical data by summative parameters (and also by tables and plots).

## Standard functions defined in math unit

The unit `math` of the run-time library provides a plethora of routines for descriptive statistics.

These functions expect an array of predefined length (e.g. `array[1..100] of float`) or a 0-based open array (e.g. `array of extended`) with subsequent call of the `setLength` procedure.

## Custom functions

Some functions aren't defined in the RTL. The subsequent section lists source code of some commonly used measures for centrality and dispersion. Where not otherwise specified the code is provided with a BSD license.

Together with other useful statistical code expanded and thoroughly tested versions of these functions are also available in the QUANTUM SALIS project.

### Median

The term median denotes the 50% quantile of a sample, i.e. the value separating the higher half of a data vector from its lower half. It can be calculated with:

```type
TExtArray = array of Extended;

// Based on Shell Sort - avoiding recursion allows for sorting
// of very large arrays, too
function sortExtArray(const data: TExtArray): TExtArray;
var
data2: TExtArray;
arrayLength, i, j, k: longint;
h: extended;
begin
arrayLength := high(data);
data2 := copy(data, 0, arrayLength + 1);
k := arrayLength div 2;
while k > 0 do
begin
for i := 0 to arrayLength - k do
begin
j := i;
while (j >= 0) and (data2[j] > data2[j + k]) do
begin
h := data2[j];
data2[j] := data2[j + k];
data2[j + k] := h;
if j > k then
dec(j, k)
else
j := 0;
end;
end;
k := k div 2
end;
result := data2;
end;

function median(const data: TExtArray): extended;
var
centralElement: integer;
sortedData: TExtArray;
begin
sortedData := sortExtArray(data);
centralElement := length(sortedData) div 2;
if odd(length(sortedData)) then
result := sortedData[centralElement]
else
result := (sortedData[centralElement - 1] + sortedData[centralElement]) / 2;
end;
```

Of course, the function `sortExtArray` may be replaced with another sorting algorithm, e.g. QuickSort. The Shell Sort algorithm presented here has the advantage that it is able to sort very large vectors even on machines with a very small amount of memory (albeit with the expense of slightly reduced speed compared to QuickSort).

### Standard error of the mean

The standard error of the mean (SEM) is a measure that estimates how precisely the true mean of the population can be known.

Calculation of SEM is simple:

```function sem(const data: array of Extended): extended;
begin
sem := stddev(data) / sqrt(length(data));
end;
```

### Coefficient of variation

The coefficient of variation (CoV or CV), also known as relative standard deviation (RSD), is a measure of dispersion that is standardised with respect to the data's mean.

It can be calculated with:

```{**
calculates the coefficient of variation (CV or CoV) of a vector of extended
}
function cv(const data: array of Extended): extended;
begin
result := stddev(data) / mean(data);
end;
```