Important Analytic Considerations and Limitations Regarding Environmental Chemical Data Analyses
Before beginning an analysis of environmental chemical data, it is useful to have a basic understanding of some key analytic principles that may affect the results. The module reviews: limitations to consider when analyzing NHANES environmental chemical data; how to address the limit of detection aspect of laboratory measures; and NHANES environmental subsamples and appropriate subsample weights.
Task 1: Explain Concentrations of Chemicals in Blood and Urine Compared to Concentrations of Chemicals in the Environment
When analyzing NHANES data of environmental chemicals, it is important to consider that blood and urine concentrations of the chemicals measured in NHANES participants may not represent the background concentrations of these chemicals that are present in air, water, soil, or dust in the United States.
- Key Concepts about Concentations of Chemicals in Blood and Urine Compared to Concentrations of Chemicals in the Environment
Task 2: Limit of Detection (LOD) Considerations
Unlike physical measures such as height and weight, which have a measured value greater than zero, laboratory measures can be limited by assay technical limitations or limits on the quantity of specimen available for measurement. For most laboratory measures of environmental chemical concentrations, a lower Limit of Detection (LOD) exists for the reported laboratory finding. In rare cases, an upper detection limit bound may exist beyond which it is not possible to accurately measure concentrations of chemical with the particular assay.
- Key Concepts about Limit of Detection (LOD) Considerations
- How to Determine Different Limit of Detection Fill Values in NHANES Environmental Chemical Data
Task 3: Subsamples and Weights Associated with Environmental Chemical Data
As with all of the NHANES data, sample weights should be used to generate national estimates from the NHANES sample of the United States. Careful attention should occur to note the proportion of the NHANES sample that is used to collect specimens for environmental chemical testing (i.e. full sample, ½ sample, or 1/3 sample, which is the most common). For a various reasons, only some, but not all, participants are selected for environmental chemical measures. The subsamples selected for these components are chosen at random with a specified sampling fraction (for example, 1/2 or 1/3 of the total examined group) according to the protocol for that component. Each component subsample has its own designated weight, which accounts for the additional probability of selection into the subsample component, as well as the additional nonresponse.
Task 4: Other Data Considerations and Limitations Related to Environmental Chemical Data
There are several other considerations regarding environmental chemical data that should be considered before analyzing the data.