Topic: DMD0117
STDEVR - Sample Standard Deviation of a Range
This instruction can only be used in the Expression parameter of the Calculate Expression (MATH) instruction.
The Sample Standard Deviation of a Range (STDEVR) math function calculates the Sample Standard Deviation across a Range of values in a data-block. Generally speaking, standard deviation shows how much variation or "dispersion" exists from the average (mean, or expected value). A low standard deviation indicates that the data points tend to be very close to the mean, whereas high standard deviation indicates that the data points are spread out over a large range of values.
This is the mathematical formula used, where N denotes the number of data points, xi denotes each individual data point, and x-bar denotes the average (or mean) of the N data points.
The Sample Standard Deviation is typically used when a sample of data from a larger population is available. Otherwise the Population Standard Deviation of a Range (STDEVPR) math function should be used.
Note: Use the F9 key or click the 'three dot box' at the right edge of the parameter field to open the Default Element Selection Tool (the Element Picker or the Element Browser) or use the Down-Arrow key (Auto-Complete) on any parameter field to see a complete list of the memory locations that are valid for that parameter of the instruction.
Result specifies a location to store the result of the calculation. This function always evaluates to a floating-point values, so selecting a writable Real (floating point) location is the proper choice.
The Expression parameter consists of all the text between the bounding parentheses. The parameter list must consist of 2 parameters separated by a comma. Each parameter is an expression which can be a constant value, a bit or numeric memory location, an arithmetic expression, and / or mathematical function or other expressions up to a maximum of 1024 characters. Expressions can be nested, so use of parentheses is recommended to ensure proper evaluation order.
This function has two parameters separated by a comma:
The first parameter specifies the first location in the range of values. This can be any readable any numeric or bit data block location, including casts and structure members (fields of heap structures are not supported).
The second parameter specifies the number of successive locations that are in the range. Note that this value is a count value, it does NOT specify the ending location. This can be any constant value from 1 to 65535, or a bit or numeric memory location, an arithmetic expression, and / or mathematical function or other expression. Expressions can be nested, so use of parentheses is recommended to ensure proper evaluation order.
If the second parameter (Range) is 0 or 1, the result will be set to 0 and ST132 (Argument Out of Range) will be ON.
If the second parameter (Range) exceeds the size of the data block , the result will be calculated using the values up to the last variable in the block and ST132 (Argument Out of Range) will be ON. For example, assume the block of WX locations has a range of WX0 to WX255. The result of STDEVR( WX250, 10 ) would be the Sample Standard Deviation of the 6 values in WX250 through WX255, and ST132 (Argument Out of Range) will be ON.
See Also:
STDEVR - Sample Standard Deviation of a Range
STDEVPR - Population Standard Deviation of a Range
Related Topics:
AVGR - Average of a Range of Values
MAXR - Maximum Value Across a Range of Values
MINR - Minimum Value Across a Range of Values
SUMR - Sum of a Range of Values
Rung Example:
