I have got a simple questionaire hosted on the internet. It simply has a list of questions with ten multiple choice answers per question. I am looking for to ensure that the data is as genuine as possible, i.e. ensuring users only answer the question once. This is my main concern, but any other methods people use to ensure the quality of their data gained via the internet I would be interested in.
The website uses a SQL database, and uses PHP as well.
Any ideas, and lets hear them