My main point was that the validity of a questionnaire relies first and foremost upon reliability. If our questionnaire cannot be shown to be reliable, then we have no discussion about validity. For me personally I worked hard to maintain reliability when using questionnaires but I still can't see how its entirely reliable as the variable tends to be the clients current perception, and latterly the perception they've been taught by a practitioner.
Validity refers if our questionnaire measures what we intended it to measure. That is my issue, how can we know that the results haven't been skewed? I've seen a client fill one in and was almost certain he'd filled it in based upon his "current" understanding of food and health. He naturally became a slow oxidizer (carb type), a few days later after some Weston A. Price style "education" he retested as a fast oxidizer. Nothing wrong with that but how can we prove the results are not skewed by someone reading a book or chatting with a practitioner? I'm not trying to suggest that practitioners are the only ones guilty of imparting their views, its natural to advertise your beliefs, its part of sales and you can rest assured that Hovis will continue to try and convince people that fibre is of upmost importance for health.
Ledley (1956) found that questionnaires could prove a useful tool to match a respective set of symptoms with a diagnosis. So it is my suggestion that the use of a questionnaire can prove useful as a preliminary appraisal of a clients health (Brodman, Erdmann, Lorge, Wolff & Todd, 1951). After that the practitioner should be seeking to make a more thorough investigation using validated tools and methods.