Questionnaire responses need to be converted to numerical or other symbols in order for data to be analysed by computer. Instructions for coding should be written as the questionnaire is being developed. The coding scheme should be as simple as possible and consistent rules should be followed. For the coding scheme, numbers are preferable to letters as data are easier to handle in computer analyses and require less storage space.

If a paper form is used, the inclusion of a column of boxes on the right hand side of the form headed "for office use" will enable coding of the information to be carried out easily and quickly. To use the same form for data collection and coding minimises transcription errors. The size of the coding boxes should be large enough such that coded data are legible. Using a consistent coding scheme minimises coding errors. For example, if the answers "no" and "yes" to one question are coded 0 and 1 respectively, then all similar questions should be coded in the same way. None of the coding boxes should be left blank. Thus, there should be a missing value code for every item and a "not applicable" code where appropriate. For example, if a subject never eats a breakfast cereal, a question on the type of cereal is not applicable, but a code is needed for "type of cereal" so that the coding box is not left blank.