One disadvantage of this method is the problem of meals consumed away from home, which it may not be possible to weigh. In these cases, subjects should be asked to give a complete description of the meal and to estimate the quantities in household measures. For snack items such as cakes or filled rolls, subjects could be asked to state the place of purchase so that a sample can be bought and weighed. Alternatively, it may be possible to issue subjects with a camera with a built-in flash and ask them to photograph meals consumed away from home. Weights of foods can then be estimated with the aid of standard slides. Comparison of intake estimates with those from weighed intake records has shown that correlations are high (0.8 - 0.9).
The response rate for the weighed intake method may be lower than most other dietary survey methods, as it requires a high degree of subject co-operation, although response rates of 90% have been achieved in population surveys. A high response rate is more easily obtained if subjects feel that the survey will be of benefit to them or their peers, e.g. a coronary heart disease study among a high risk group such as middle-aged men.
A major disadvantage of this method is its high cost. This is because it is labour-intensive in terms of both data collection and data processing. Another disadvantage is that subjects may alter their diet during the survey period, or that some items of food and drink consumed may be omitted from the record.
For a population survey, at least three visits to each subject's home are needed in order to ensure that records are completed satisfactorily. The first visit is used to explain the purpose of the study and demonstrate the method of weighing. It is important to stress that the record should be of their usual diet and that they should therefore not alter their diet during the survey period. An atypical period containing dinner parties or other celebrations should be avoided. Subjects should be revisited the following day, the first day of weighing, to check that the technique has been understood and is being carried out correctly. This visit is of great importance, not only to clear up any problems that may have arisen, but also in maintaining enthusiasm. (If subjects are simply given a telephone number to ring if they have a problem, few will use it and the result may be a pile of incomplete or unusable records). The most frequent errors encountered during the second visit are that a single weight is given for "bread and butter" and for "tea with milk and sugar". There may also be confusion about how to weigh and record composite dishes, such as stews and casseroles. Additional visits should be made during the survey period if necessary.
At the final visit, the record should be thoroughly checked with the subject to ensure that all food and drink consumed has been recorded, and that sufficient detail has been given about the foods to enable calculation of nutrient intakes. If items have been omitted from the record, these tend to be beverages such as tea, coffee, fruit juices and alcoholic drinks or snack items such as biscuits, sweets and crisps.An example of insufficient detail recorded is "bread roll" - it is necessary to know whether the roll is white, brown or wholemeal and whether it is soft or crusty. Also, recipes or brand names for composite dishes may not always be given. If you are interested in the fatty acid composition of the diet, you need to know what kind of fat is used for frying and for any homemade cakes, pastries, etc.
Having collected the records, data must then be entered into a computer for nutritional analysis (see WISP). This should ideally be done within a few days of collecting the record. The reasons for this are firstly that, since cumulative weights are recorded, it is not always apparent during the home visits that weights for some items may seem unbelievably large or small. Secondly, it is possible that a missing detail about a food item was not observed at the final visit. Processing of data within a few days will thus enable clarification of these points by questioning the subject further.
