This example will explain how to do a BMI calculation using number fields and functions

1. Add two Number fields to capture the participant's height and weight respectively. 

  • Rename the fields to e.g. 'height' and 'weight'
  • For each of these, select the 'Decimal' answer format on the Basics tab.

2. Add a Calculated Value field to calculate (but not display) the participant's BMI using the responses to the two Number fields added above and BMI = weight (kg) / (height (m))2

  • Type in the formula as "=$weight / pow($height, 2)"
  • Select the 'Decimal' data type.

3. If you want to show the value of the BMI calculation to the fieldworker, add another field (e.g. an Information field) and reference the Calculated Value field name. 

  • In this case, the "round" function was used to round the answer to one decimal place: simply type "=round($bmi_calculation, 1)" in the display text.

On the fieldworker's handset, it will look as follows. 

Note: If nothing was calculated (e.g. the field(s) on which the calculation are based was skipped), "NaN" (Not a Number) will be shown instead of a number.

The calculated value can also be used as a source field to base skip logic on. For example, certain fields should only be asked if the participant's BMI is equal to or greater than 23. 

  • This field will be skipped if the value for BMI as calculated above is less than 23.