A large multicenter case-control study investigates the association between postmenopausal estrogen therapy and endometrial cancer risk. Investigators stratify participants by body mass index (BMI) into three groups: normal weight (BMI < 25), overweight (BMI 25–29.9), and obese (BMI ≥ 30). The following odds ratios (ORs) and 95% confidence intervals (CIs) are reported for estrogen therapy users versus nonusers:

• Normal weight: OR 3.0 (95% CI 2.1–4.2) • Overweight: OR 1.8 (95% CI 1.3–2.5) • Obese: OR 1.1 (95% CI 0.8–1.5)

Which of the following terms best describes the observed variation in the effect of estrogen therapy on endometrial cancer risk across BMI strata?