4.1.3.2.8 MeanError

MeanError function

Prototype:	function MeanError (const StandardDeviation :extended; const NSample :integer) : extended;
Description:	Calculates maximal accepted error of estimate of variable with specified number of observations and standard deviation
Syntax:	variable := MeanError (StandardDeviation, NSample);
	Part	Description
	variable	A variable declared as type of extended
	StandardDeviation	Sample standard deviation
	NSample	Number of observations in sample


Return value:	Mean error
Note:	Calculations are based on the presumption of unknown population standard deviation, i.e. quantiles of Students distribution are used.
Definition:	Quantile of Students distribution (alpha 5 %) * StandardError

Example:

Task:

Calculate mean error of DBH.

Data stored in the database.

Script:

var

tab : TTableWrapper;

attributeName: string;

nullAsZero: boolean;

x,e: extended;

begin

tab:= Trees.table;

attributeName:= 'DBH_mm';

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

x:= StandardDeviation(tab, attributeName, nullAsZero);

e:= MeanError(x, Trees.RecordCount); // Trees.RecordCount returns number of records

//e:= MeanError(StandardDeviation(tab, attributeName, nullAsZero), Trees.RecordCount); //this is same as previous line

ShowMessage(FloatToStr(e));

end.

Output in Field-Map Data Collector:

mean error of DBH