Skip directly to search Skip directly to A to Z list Skip directly to navigation Skip directly to site content Skip directly to page options
CDC Home

NHANES Environmental Chemical Data Tutorial

Review Data & Create New Variables


Reviewing NHANES environmental chemical data and creating new variables may be necessary before you can use the variables in the dataset. NHANES environmental chemical data may need to be adjusted if the dataset has missing data or outliers. Depending on the purpose of your analysis, you may also need to create new variables (e.g., to create a category variable based on level of detection).  


Task 1: Identify, Recode, and Evaluate Missing Data

Missing values may distort your analysis results. You must evaluate the extent of missing data in your dataset to determine whether the data are useable without additional re-weighting for item non-response.


Task 2: Check Distributions and Describe the Impact of Influential Outliers

Before you analyze environmental chemical data, it is very important that you check the distribution and normality of the data, identify outliers, and determine how outliers might affect your analysis.


Task 3: Check for Data Symmetry

Many statistical procedures are based on the assumption that data are normally distributed, and therefore, symmetrically distributed. However, the distributions of environmental chemical concentrations in blood or urine are often skewed.


Task 4: Create New Variables

Recoding is an important step for preparing an analytic dataset. You may want to recode variables or create new variables that fit your analytic needs.



Contact Us:
  • National Center for Health Statistics
    3311 Toledo Rd
    Hyattsville, MD 20782
  • 1 (800) 232-4636
  • The U.S. Government's Official Web PortalDepartment of Health and Human Services
Centers for Disease Control and Prevention   1600 Clifton Road Atlanta, GA 30329-4027, USA
800-CDC-INFO (800-232-4636) TTY: (888) 232-6348 - Contact CDC–INFO
A-Z Index
  1. A
  2. B
  3. C
  4. D
  5. E
  6. F
  7. G
  8. H
  9. I
  10. J
  11. K
  12. L
  13. M
  14. N
  15. O
  16. P
  17. Q
  18. R
  19. S
  20. T
  21. U
  22. V
  23. W
  24. X
  25. Y
  26. Z
  27. #