Hello,

I'm working on a survey with patient data and have two numeric fields for height and weight. I'd like to set the default value of another field, BMI, with the BMI calculation using those two fields. So far I haven't had any luck doing so. Could someone point me to resources on how to do this?

Hi,
in my opinion you do not want to set the BMI value as default of another field?
Why should you do that?
That the respondent may change it?

I think you want to store this value in the dataset, maybe display it to the respondent.

So create a question of type equation, where you enter the formula
{weight/(height*height)}
where "weight" and "height are the question codes.
If the codes of these two questions are "Q1" and "Q2", it is
{Q1/(Q2*Q2)}
Hopefully with the correct units.

Have a look at this sample survey
www.limesurvey.org/manual/Expression_Man...loring_and_Equations

and read about Expression Manager. (which in version 4.x. is called "Expression script")
www.limesurvey.org/manual/Expression_Manager

Joffm

And if you really want to prefill a third question, change the equation to
{Q3=weight/(height*height)}

Thank you so much for your help! I have never used equation question types before, and was just curious - in order to save just the BMI value, would I copy {weight/(height*height)} in the Equation field under Logic > Equation? I'm just trying to ensure that's the value saved.

in the Equation field under Logic > Equation?

Do you mean the "question validation equation"?
This is something different. Here you enter an equation to validate the question, e.g. (Q1<10) OR (Q1>60), to get only very small or very high ages, or whatever.

Your equation you enter in the question text of the equation question.


Joffm