Successful questionnaire design
Write the questions
Avoid using double-barrelled questions
Ask only one question at a time
“Double-barrelled” questions that might cause the respondent to not know how to answer.
- Do you like cats and dogs? ---- YES  / NO 
How do you answer this question if you like cats but you do not like dogs?
There is the likelihood that the respondent may leave the answer blank or choose one of the options at random just to complete the questionnaire. Either way the answer will be unreliable.
This is actually two questions and would be better written as such.
- Do you like dogs? ----- YES  / NO 
- Do you like cats? ----- YES  / NO 
You may think this is too obvious an error to make but it happens all the time.
The next subchapter introduces the 'Other' category.