| Food Composition Data: A User's Perspective (1987) |
|A system for evaluating the quality of published nutrient data: Selenium, a test case|
Calculation of the mean SE value and confidence code
The quality index (QI), a measure of the overall quality of data from a single study, was derived in one of two ways: (a) when analytical method is rated 0 or when three or more categories are rated 0, the QI for that study becomes 0; or (b) when those conditions do not exist, the ratings for each of the five categories are averaged. Thus, the QI can range from 0 to a maximum of 3.0.
The mean Se values reported on a fresh weight basis from those studies which have a QI equal to or greater than 1.0 are averaged together to obtain a mean Se value. Values reported on a dry weight basis cannot be combined with values reported on a fresh weight basis, i.e. foods as eaten, unless moisture levels are included with the dry values to permit calculation back to fresh weight. If no moisture levels are reported, dry values are excluded regardless of their QI.
A confidence code (CC), assigned to the mean Se value for each food, indicates the degree of confidence a user can have in the mean value. It is determined by summing the QIs equal to or greater than 1.0 among the various studies evaluated for a given food item, and then referring to table 3 for the corresponding CC. The basis for the CC is the necessity of confirming the results of one report by other investigators in order to be considered valid. Thus, data from a minimum of three studies with a sum of QIs of 6.2 are required for a mean value to be assigned a CC of a. The cut-off point between b and c of 3.4 was made by dividing approximately equally all the possible sums of QIs, i.e. from 1.0 to 6.0.