A History of the Ministry of Information, 1939-46

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Adequacy of Method

It was realised that homely measures would be less accurate than weighing. The point was whether the homely measures would give values near enough reality to be of practical use in the estimation of the nutrient intake of the investigated group.

Many psychological experiments (described in any ordinary textbook of experimental psychology) show that people are able to estimate within a certain margin of error the size of a given object by comparing it with a standard size. Dr. Bransby had carried out some special experiments with domestic science students in which they compared the size of certain pieces of foods with standard sizes (the results will be published elsewhere) and had found that though individual students over and under-estimated, the average value for the group obtained from homely measures differs only slightly from the values obtained from weighing the food.

In order to see whether ordinary housewives would have the same ability as students in measuring, 18 housewives of different ages and economic groups, some with families, some without, were selected. Each of these for a week measured her own food, first in homely measures and then by weighing it. The results, comparing the nutrient values obtained from each, are shown in the following table:

Table 1
Nutrients estimated from measured diet * expressed as percentage of nutrients estimated from weighed diets * Lowest under-estimate and highest over-estimate
Calories 109 80 - 140
C.H.O. 107 76 - 140
Protein: Animal 113 81 - 174
Vegetable 105 73 - 135
Calcium 108 92 - 153
Iron 112 87 - 153
Vit. A 107 59 - 167
Carotene 104 69 - 127
Vit. B1 104 71 - 138
Vit. C 103 80 - 138
Fat 112 78 - 156
Sample - Weekly diets of 18 housewives.

* For purposes of simplification henceforward, the method of measuring Food intake in homely measures will be referred to as a “Measured diet”; the method of measuring Food intake by weighing will be referred to as a “Weighed diet”.

The average value of quantities obtained from the measured diets is fairly near to the one obtained from the weighed diets. A further analysis of the misjudgments combined with the observations of the interviewers when supervising the Informant, showed that the same individual over and under-estimates at different occasions, though a slight tendency to over-estimate is noticeable. There are two reasons for this. The standards given in the poster were rough; no account was taken of such phenomena as the effects of contrast, background, etc., or the influence of angle and distance from which the Informant would view the poster when making comparisons. In future surveys the posters will be better devised and the interviewer given rules as to what place and distance from the serving housewife the poster should be placed.

The other cause of over-estimation is a general tendency of housewives to over-estimate their purchases and servings of food, as we have found in many other food investigations.

On the whole these experiments seemed to justify the belief that the method would be worth a trial, particularly as it would have one advantage in accuracy over the weighing method, which in 1944 showed that any nutrient intake calculated with the help of tables from the food quantities consumed, can be seriously biased by the errors arising out of the different cooking methods, i.e. different ingredients placed in made-up dishes, porridge, milk puddings, etc. This error decreases with an increasing sample where the errors can cancel each other out. Measured diets are based on larger samples: the nutrient intake obtained from them will, therefore, in this respect be more accurate than from weighed diets.

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