"Cannot convert null to logical" --when I know there are nulls and I want to keep them

[JoeCrozier]

Hello, I'm having a problem converting nulls to logical, and I know there are already a lot of questions like this online:

But I believe most of those come from the user not knowing where the nulls are.  I know where mine are, and I want to do something with them.  

Here's my code:

And as you can see, what I WANT my code to do is create a column that says a visit is late if the 'planned_late_days' is negative, say its on time if the number is positive, or say its not documented if that column was null (which would have happened if one of the columns needed for that calculation was blank).

Its mostly working, except when 'planned_late_days' is null, instead of saying its not documented, it throws an error.   Is there a workaround?  I dont want to filter out the nulls.

Thank you

[Anonymous · Accepted solution]

Try changing it from

else if [Column] = null then 

to

else if is null [column] then

Also note that you could eliminate the third leg altogether, since you have all the possibilities covered already, so

if negative then this, if positive then that, else null

--Nate