One of the features of NDSR that makes it the choice of researchers is the small number of missing nutrient values for foods in the database. This is an essential feature because missing nutrient values for foods are ultimately calculated as zeros in nutrient intake estimates.
To minimize the number of missing values, NCC uses several standardized procedures to impute or logically calculate an estimation. These procedures are described in Procedures for estimating nutrient values for food composition databases. To summarize, one of the following procedures is employed to estimate a nutrient value for a food when an analytic value is not available from the USDA Nutrient Data Laboratory or cannot be found in the literature:
- Value from a different but similar food is used. (e.g., missing nutrient for wild duck may be judged to be the same as known nutrient in domestic duck)
- Value for another form of the same food is calculated (e.g., convert from raw to cooked values using retention factors)
- Calculate values from other components in the same food (e.g., estimate beta-carotene from Vitamin A)
- Calculate value from household recipes or commercial food product formulations for multicomponent foods (e.g., Manufacturers may provide ingredient listing and macronutrient composition of product. From the macro-nutrients, NCC database scientists estimate the amount of each ingredient. Based on the estimated amounts of the ingredients, micro-nutrients can be imputed for the product.)
Missing values are allowed in the database for foods that are consumed in very small quantities, such as spices, or where there are no data to indicate whether the nutrient exists in the food.
Some nutrients have no missing values, but a high percentage of imputed values. An example is Vitamin A which is calculated from provitamin A carotenoids and retinol.
In future versions of the database, missing and estimated values will be replaced by analytic values as they become available.