Survey Analysis : average with Text & Numerical data

Hello, 

I would like to ask you for some help analyzing a survey that uses text and numerical data. 

Indeed, I am designing a survey and respondents can rate a service (from 1 to 5) and they can also say "Not applicable". I have a column that contains the id of the respondent, it's nice for filtering. But, regarding the mix of numbers and text is that the outcome is that my column contains, for instance : 

For analysis I would like to :

- count the number of not applicable, so just count a field; 

-calculate the numerical average of this column.

I have created a measure, which calculates the average but how can I exclude in DAX the not applicable? And, are there any other options? 

It's for a KPI analysis. 

Thank you for your help, and have a nice da!