Conicity Index: NHANES Microdata Derivation
NHANES measures waist circumference (BMXWAIST), standing height (BMXHT), and weight (BMXWT) in the same examination. FitnessNorms used those measurements to calculate the index introduced by Valdez (1991).
- Formula:
CI = WC / (0.109 × √(weight_kg / height_m)), with waist circumference and height in metres and weight in kilograms. A value near 1 indicates a waist close to the cylindrical reference; higher values indicate a larger waist relative to that reference. - Data: NHANES 2015-2016 (
BMX_I / DEMO_I), 2017-March 2020 (P_BMX / P_DEMO), and 2021-2023 (BMX_L / DEMO_L). Adults aged 20 and over with complete inputs were included; participants recorded as pregnant (RIDEXPRG = 1) were excluded. The final sample is n = 18,716. - Weights: MEC weights (
WTMEC2YR, orWTMECPRPfor 2017-March 2020) were scaled by each period's duration: 2/7.2, 3.2/7.2, and 2/7.2. CDC documents the 3.2-year pre-pandemic file and duration-based adjustment when combining it with two-year cycles. Applying the same duration principle across the 2021-2023 release is a FitnessNorms choice; CDC cautions analysts about combining data across the pandemic discontinuity. - Sensitivity gate: every pooled percentile was compared with the 2021-2023-only result. Thirteen of 130 cells differed by more than 0.02 index units, with a maximum absolute difference of 0.037. The cells occur in both sexes, across nine age brackets and all five quantiles, with mixed direction. Each accepted cell is listed in the derivation script; an unlisted breach prevents the pooled artifact from being written.
- Why pool: the pooled table provides larger cells and more stable tails. It should be read as a reference spanning the included survey periods, not as an estimate for a single current year.
- Quantiles: weighted empirical P5, P25, P50, P75, and P95 values were calculated separately for each sex and five-year age bracket from 20-24 through 80+. Linear interpolation was applied to the weighted cumulative distribution. Survey-design standard errors were not calculated.
Interpretation and limits
No universal clinical Conicity Index cut-off is established. Published thresholds vary by population and outcome, so FitnessNorms reports distribution percentiles rather than risk categories. The public NHANES age variable is top-coded at 80, and the 80+ bracket combines everyone recorded at that age or older.
This is an internal FitnessNorms derivation rather than a peer-reviewed percentile study. See the reference page for a compact provenance and limitations record.