SYNTAX:

GDEV(VarNum,GroupOffset/StartDate,Grouping/EndDate) 

VarNum:  The variable number that refers to the values you want to look at.

GroupOffset/StartDate: Used with the Grouping parameter to set the dates to calculate the stat for.  Sets the number of Groups (set by the Grouping Parameter) to be added to set the dates.  See examples below. If this field is a date string (i.e. "07/16/2004") or refers to a cell with a date in it, it is assumed to be the start date to calculate the stat for.

Grouping/EndDate: Used to set which dates to calculate the stat for.  If the Grouping is a date literal string (i.e. "07/16/2004") or refers to a cell with a date in it, it is assumed to be the end date to calculate the stat for. Common Groupings are:

"D" Days
“M” Monthly 
"R" Report Dates
Click here for available Grouping options

NOTES:

The standard deviation is calculated using values from the start date to the end date, not all values ever collected.

EXAMPLES: