Time series specific value

[pgrign]

Hi everybody,

I have an issue that I guess it's really easy to solve, but I can't figure it out.

In a time series with quarterly data I have a regression to run, and I want the program to choose the regressor: i.e., if the value corrisponding to 2017Q1
in series "A" is "NA" -> use series "B" as regressor.

So I would use the IF statement. The thing is that, how do I check if in 2017Q1 I have a numerical value or "NA"?

Thank you

[EViews Gareth]

@elem