valueSD()

float valueSD(string question)

float valueSD(string question, string|array items)

float valueSD(array variables)

Calculates the standard deviation of the answer codes for all items in a question (e.g. a scale), for selected items, or for a list of variables.

• question – ID of a question (as a string)
• items – A list (string or array) of items
• variables – A list (array) of variable IDs as stated in the Variables Overview

Note: This works in the same way as valueSum() – the parameters are described in detail in this chapter with various examples.

The following PHP code identifies participants who always selected the same value (straight-lining) in the scale “AB01”, and sends them to the end of the questionnaire immediately.

$sd = valueSD('AB01');
if ($sd == 0) {
  goToToPage('end');
}

The following PHP code displays a message to participants who predominately chose the same value in the scale “AB02”.

$sd = valueSD('AB02');
if ($sd < 0.25) {
  text('think more');
}

Note: The standard deviation of a scale is not a valid indication of data quality in a questionnaire. ¹⁾. Only use the above examples with the utmost caution.