We recommend including an N/A ("not applicable") option in your rating scale. This lets feedback providers indicate that they don't feel they are able to answer a question fairly.
To add this:
Open your project
Click Design on the left-hand side
Click Edit questionnaire
Click Edit rating scale in the orange menu on the left (see image below)
Click Edit scale
Click Add N/A
You can optionally choose to give the N/A option a longer name e.g. 'Not observed' or 'I don't know'.
When done, click to save the changes