Not sure if that title best describes what I'm trying to do but I hope that someone can help me with the coding for this problem.

I'll be asking about a series of questions for which data may not be available, e.g. height, weight, shoe size

We like to have a response for every question, i.e. no blank values. For dropdown fields, we'll include an "unknown" value, but to handle single-item fields, I am thinking of including a field (multi-select or checkbox) that allows people indicate that data for several variables is unknown, e.g. Are any data unknown? Option 1: Height; Option 2: Weight; Option 3: Shoe Size.For each of the items selected, I want to have an action triggered (on user input) that then fills the original question with a value, e.g. user selects Options 1 and 2 to the multi-select unknown question and the original Height and Weight single-item fields are filled with the values of "999" (which is just a filler value).

Anyone have thoughts about how to code this?

Thanks!