Food intake questionnaires have been widely used in large-scale population surveys. Questions may refer to frequency of consumption, e.g. "On how many days per week do you usually eat fresh fruit?" or they may refer to quantity, e.g. "How many eggs do you usually eat per week?". Questionnaires are usually termed "food frequency" or "semi-quantitative food frequency" depending upon whether questions on the amounts of foods consumed are included. Food intake questionnaires have several advantages over other dietary survey methods:

• Quick and inexpensive
• Readily survey large number of subjects
• Readily survey wide geograhical area (forms returned by post or interview completed by phone or internet)
• Observer bias minimised
• Coding of data is simple
• Can provide same answers as more complex methods, for certain types of survey
• May be more representative of usual intake than detailed studies of a few days

If you wish to conduct any of the following types of study, a food intake questionnaire such as DIETQ will be a suitable dietary survey method, giving the same answers as more complex methods such as the weighed inventory:

• Calculate mean nutrient intakes for a group of subjects
• Compare mean nutrient intakes of those who have a particular disease with those who do not, e.g. subjects with ischaemic heart disease versus controls
• Compare mean nutrient intakes of certain types of subjects with others, e.g. vegetarians versus omnivores
• Compare mean nutrient intakes of those who have been given dietary advice with those who have not
• Compare mean nutrient intakes of a group of subjects before and after dietary advice
• Estimate the amount of certain foods, e.g. fruit and vegetables or fish, consumed by a group of subjects

If you wish to investigate whether there is a correlation between particular nutrients and say blood lipids or blood pressure, then the ability of a questionnaire to detect such assocations will depend upon the questionnaire used, how responses are converted to nutrient intakes and the food or nutrient of interest. Some nutrients are much easier to study by a food intake questionnaire than others. For example, intake of fibre, vitamin C or alcohol can be more readily estimated by questionnaire than many other nutrients. This is because fibre, vitamin C or alcohol are present in relatively few foods and people can estimate how often they eat these foods reasonably well. Day to day variation in intake of a nutrient is also a factor in how well that nutrient can be estimated by questionnaire - nutrients where intakes do not vary greatly from day to day can be more readily estimated than those for which intakes vary considerably from day to day.

Studies comparing people's estimates of how often they consume various foods with actual consumption from 30-day food intake records have shown good agreement between estimated frequencies and actual frequencies. The degree of association depends on the particular food item: foods that are eaten habitually tend to be better estimated than those eaten relatively infrequently.

Note: if using a questionnaire that has been developed and validated by someone else, ensure that you have the computer software package to calculate nutrient intakes from the questionnaire as well as the questionnaire itself. This is because the validity of the questionnaire depends not only on the questions but also how the responses are converted to nutrient intakes. If you do not have the computer software package that converts respones to nutrient intakes, you cannot assume that the validity of the questionnaire will be the same.

Q-Builder is a questionnaire development tool from Tinuviel Software. This program can be used to develop any type of questionnaire. You can collect data using a paper form, computer-based interview or web interview. For food intake questionnaires, nutrient intakes can also be calculated.