Subjects are asked to recall all food and drink consumed over a specified period, usually 24 hours. Questions are asked about each eating occasion, starting with breakast going through the entire day and including night time. (The latter is particularly important for night shift workers). Details are asked about each food and drink consumed. For example, for bread, questions should be asked about the type of bread consumed and the type of spread added. For meat, questions should be asked about the type of meat, how it was cooked and whether the fat on the meat was eaten. For fish, questions should be asked about the type of fish, how it was cooked and, if fried, what type of fat was added. For vegetables, questions should be asked about whether salt is added in cooking if one is interested in sodium intake. If baked potato is eaten, questions should asked about whether the skin was eaten and what type of fat (if any) was added. For drinks such as tea or coffee, questions should be asked about the type of milk added (if any) and whether sugar was added. Where appropriate, questions should be asked about whether foods are reduced calorie, reduced sugar, reduced fat, etc. Only by asking details about each food consumed can an appropriate nutrient composition be assigned to each food.

Appropriate portion weights need also to be assigned to each food. This can be done in a number of ways. For example, questions could be asked about the number of slices of bread and an average weight used for one slice. Questions could be asked about the number of teaspoons of sugar added to a food or drink and an average weight used for one teaspoon of sugar. For foods such as meat or fish, average portion weights could be used or portion weights asessed with the aid of food photographs or food models.

The recall method requires an experienced interviewer, particularly if periods longer than 24 hrs are used. For periods longer than 24hrs, details of the food and drink consumed can be obtained by asking about events on the day in question. Ensuring consistency can be difficult, even if there is only one interviewer. Another disadvantage of this method, particularly for periods of more than 24 hrs, is its reliance on memory. This may make it an unsuitable method for certain groups, e.g. the elderly.

The 24-hr recall method has been widely used in population studies. However, nutrient intakes can vary considerably from day to day and thus a single day is unlikely to be representative of an individual's usual intake. An alternative use of the recall method is to obtain 24-hour recall data for each subject on several occasions. This better enables assessment of habitual diet, but means that each subject has to be interviewed on several occasions.

FOOD 24 is an expert system for 24 hour recall interview and nutrition analysis. The questions asked are dependent upon responses to previous questions. Responses are converted to food codes and weights automatically, enhancing the speed of data collection and minimising variation both within and between interviewers. This program enables a 24-hour recall of food and drink intake to be completed in only 5-10 minutes, together with instantaneous nutritional analysis. The interview can be completed at a subject's home, in a clinic, or by telephone.

WISP enabes a number of 24hr recalls for each subject to be combined to make a single record of several days duration.

FOOD24 Ultimate enables one to develop 24 hour recall interviews for the internet. Interviews can be carried out on the web, allowing field workers to operate in any location with internet access or for subjects to self-administer if appropriate to the study population.