[영문]Background: Health plan evaluations typically focus on beneficiaries continuously enrolled throughout a calendar year.Objective: To examine whether the sampling strategy used in evaluating health plans affects their assessment.Research Design: We completed a cross-sectional analysis of California Medicaid managed care plans aggregating individual level data by health plan.Measures: We calculated the correlation between a health plan's annual age-sex adjusted hospitalization rate for ambulatory care sensitive (ACS) conditions sampling among beneficiaries continuously enrolled throughout the year with all health plan beneficiaries enrolled for a minimum of 1 month. For health plan beneficiaries with a minimum of 1 month of enrollment we calculated hospitalization rates in 2 ways: (1) counting only those ACS hospitalizations while enrolled in the plan and (2) counting any ACS hospitalization throughout the year regardless of whether it was during the period of plan enrollment.Subjects: California Medicaid beneficiaries 18-64 years of age in 2001.Results: Forty-four Medicaid plans representing approximately 750,000 beneficiaries were included. On average, 50% (range, 26-69% across plans) of the beneficiaries were enrolled continuously during the year. Plan rankings based on the ACS hospitalization rates for each of the 3 sampling strategies were variably correlated [Spearman correlations 0.26 (P = 0.086), 0.33 (P = 0.031), and 0.71 (P qqqlt; 0.0001) for pairwise comparisons]. The agreement among the sampling strategies in labeling a health plan as an outlier was not statistically different than random chance ([kappa] = 0.069, P = 0.21).Conclusions: Judgments regarding health plan performance are affected by limiting the evaluation sample to only those beneficiaries continuously enrolled in a health plan throughout the year. Policymakers should consider the goal of health care measurement when selecting a sampling strategy and explicitly acknowledge the bias that might be introduced by a particular approach.