| Food Composition Data: A User's Perspective (1987) |
|A system for evaluating the quality of published nutrient data: Selenium, a test case|
Table 4 provides the Se data which were collected and evaluated for three foods: white rolls, whole milk, and canned crab. These foods were selected to show varying levels of Se among foods and within foods, different combinations of ratings, the typical range of QIs, and an example for each CC.
Analyses of white rolls were reported in three references. In reference 22, analyses were carried out on samples of the same product from two different sets of cities. Zeros were assigned in two categories: in analytical quality control because of poor precision in the second set of collections in reference 23, and because of lack of any documentation in references 3 and 22; and in sampling plan because of the Canadian origin of the product in reference 3. Onehalf of the ratings were 2, while 1 and 3 were assigned less frequently. The data from all four studies (three references) were found to be acceptable, and the four Se values were used to derive the mean value of 34 µg/100g, with minimum and maximum values of 21 and 61. The CC of a based on the sum of the QIs, 6.4, indicates that one can use this value with considerable confidence.
Sample analyses of whole milk were presented in 11 references which contained 15 studies. However, data from 11 studies were judged unacceptable based on a 0 rating in analytical method: lack of documentation, lack of validation of the method for milk, or reported Se levels below the stated quantitation limit. Out of the four remaining studies [2, 3, 4, 13], data from two [2, 3] were rated 0 in sampling plan due to their Canadian origin, while all were rated 0 in analytical quality control: references 3 and 4 reported no quality-control measures, while references 2 and 13 did not address both issues of accuracy and precision. Reference 4 does not explicitly report the number of whole milk samples analysed for Se; only the total number of samples is stated (103) which includes three types of milk in addition to whole milk. Although it is likely that a large number per type was analysed, the number of samples category received a 1 due to insufficient documentation. In two studies [2, 3], only one sample (in duplicate) was analysed. The mean Se level in whole milk, based on the data from four acceptable studies, is 1.6 µg Se/100g, with a minimum and maximum of 1.1 and 2.5. The sum of QIs is 5.6, which establishes a CC of b, indicating that a user of this datum can have some confidence in the value.
Canned crab samples were analysed by two sets of workers, with a Se value from only one study  receiving an acceptable Ql. The investigators reported no analytical quality-control measures (rated 0), analysed duplicate portions of only one sample (rated 1 in number of samples), and recovered only 80 90 per cent of Se in recovery trials (minimal validation results rated I in analytical method). Thus, the concentration of 22µg Se/lOOg is based on only one sample, with ratings of 0 to 2 in the other four categories. The CC of c makes it clear that more Se data are needed on canned crab.
Table 4. Data quality evaluation for white rolls, whole milk, and canned crab
|Data quality criteria ratings||Quality
|Number of samples (actual)||Analytical Method||Sample handling||Sampling plan||Analytical Quality Control|
|Rolls, white, soft, enriched||7||2(4)||2||2||3||1||2 0a||25.50||14.06||4 composites of 3 samples each analysed|
|Rolls, white, soft, enriched||7||2(4)||2||2||3||0||1.8a||21.00||4.69|
|White rolls||23||2(4)||1||2||2||0||1 4a||29||9||No Q.C. documentation|
|Rolls, brown and serve||3||2(3)||2||2||0||0||1.2a||61||24||Canadian|
|Mean||=||34||Confidence code = a|
|Whole fluid milk||7||2(4)||2||2||3||1||2.0||0||All values < QL|
|Whole fluid milk||7||2(4)||2||2||3||0||1.8||0||All values < QL|
|Whole or skim milk||23||2(9)||0||2||2||0||0||5.9||0.8||No method validation|
|Whole milk||23||3(48)||0||2||1||0||0||5.9||1.2b||No method validation|
|Whole milk||3||2(3)||3||2||0||0||1 4a||1 5||0.2||Canadian|
|Skim or whole milk||22||3(24)||0||2||2||0||0||6.9||2.1||No method validation|
|Whole homogenized||14||1(1)||2||2||2||0||1 4a||1.2||Duplicates|
|Whole fresh milk||15||1(1)||0||2||0||0||0||0.8||1.0||Duplicates; no method validation|
|Whole fresh milk||15||1(1)||0||2||2||0||0||1.1||1.7|
|Regular milk (>3.5% fat)||4||1(?)||3||3||2||0||1.8a||2.47||103 samples, all types|
|Whole homogenized milk||2||1(1)||2||2||0||0||1.0a||1.1||Duplicates; Canadian|
|Whole milk||10||1(?)||0||0||0||0||0||1.0||Reference is abstract|
|Whole milk||1||1(?)||0||0||1||0||0||4.8b||Packaged in SD (no doc..)|
|Whole milk||1||1(?)||0||0||1||0||0||1.9b||Packaged in OR (no doc.)|
|Mean||=||1.6||Confidence code = b|
|Geisha King crab.canned||24||1(?)||1||0||2||0||0.8||51||No sample handling documentation duplicates|
|Crab. processed.packed in water||5||1(?)||1||2||2||0||1.2a||21.68|
|Mean||=||22||Confidence code = c|
a. An index > 1.0 is required for inclusion of an individual Sc value in the calculation of the mean.
b Conversion from volume to weight based on item no 01-077. "Whole Milk.'' in Us Department of Agriculture, Agriculture Handbook No. 8-/ (Science and Education
Administration USDA. Washington, D C., 1975).