The questionnaire may be either:

• A paper form
• A computer-based interview to be completed on a PC or handheld computer (PDA, e.g. Pocket PC)
• A web-based interview to be completed on the internet

The use of a paper form means that it could be sent by post or given to subjects for completion at home. Thus it is easy to survey a large geographical area. However, the disadvantage is that the form requires coding, i.e. conversion of responses to numerical or other symbols for subsequent computer analysis. This takes time, even if the coding scheme is simple. The coding must be checked and data then entered into a computer. Data entry should be done twice (dual data entry with computerised verification check)to reduce the possibility of errors in data entry. Nevertheless, poor handwriting can still result in errors - a figure may be read wrongly twice!

A computer-based interview could be done by the subjects themselves, depending upon the complexity of the questionnaire, or by an interviewer. If done by an interviewer, it culd be done in a clinic, in a subject's home or by telephone. A major advantage of a computer-based interview is that the need for coding of data, checking of coding and subsequent data entry is eliminated. An additional advantage is that responses can be range checked during the interview. This reduces the potential for gross errors. In addition, if the questionnaire is concerned with food intake, range checks can be made on the responses, thereby helping to ensure that the overall diet is "reasonable".

A web-based interview enables subjects to complete the questionnaire themseleves on the internet and the completed questionnaire to be automatically emailed to a central location. Data can then be easily imported into a software package such as Tinuviel Software's Q-Builder for subsequent analysis and/or file output. The main advantages of this method of data collection are that the need for coding of data is eliminated and that a large geographical area can be readily surveyed. However, it is important to ensure that the subjects to be surveyed have access to the internet, otherwise a high response rate for the population is not feasible.