4.1.3.2.17 SampleSize

SampleSize function

Prototype:	function SampleSize (const RelativePrecision :extended; const SampleVariance :extended; const Mean :extended; const StudentLevel :integer) : integer;
Description:	Calculates the minimum number of observations to include in a sample to achieve specified alpha of Student distribution.
Syntax:	variable := SampleSize (RelativePrecision, SampleVarince, Mean, StudentLevel);
	Part	Description
	variable	A variable declared as integer
	RelativePrecision	Number from <0;1> interval indicating relative precision of estimate
	SampleVariance	Sample variance
	Mean	Mean
	Alpha	Confidence level

Return value:	Minimal number of samples to achieve specified alpha
Definition:	[(Quantile of Students distribution ^ 2) * Variance]/ [(RelativePrecision * Mean) ^ 2]

Example:

Task:

Calculate sample size of DBH.

Data stored in the database.

Script:

var

tab : TTableWrapper;

attributeName: string;

nullAsZero: boolean;

sv, m, s: extended;

begin

tab:= Trees.table;

attributeName:= 'DBH_mm';

nullAsZero:= false; // dont want calculate with a null value

sv:= SampleVariance(tab, attributeName, nullAsZero);

m:= Mean(tab, attributeName, nullAsZero);

s:= SampleSize(0.01, sv, m, 5);

ShowMessage(FloatToStr(s));

end.

Output in Field-Map Data Collector:

sample size of DBH