NHAMCS Estimation Procedures

Statistics from the NHAMCS are derived by a multistage estimation procedure that produces essentially unbiased estimates. The estimation procedure has three basic components: 1) inflation by reciprocals of the sampling selection probabilities; 2) adjustment for nonresponse; and 3) a population weighting ratio adjustment. Beginning with 1997 data, the population weighting ratio adjustment for OPD estimates was replaced by an adjustment that controls for effects of rotating hospital sample panels into and out of the sample each year. (The full NHAMCS hospital sample is partitioned into 16 panels which are rotated into the sample over 16 periods of 4 weeks each so that only 13 panels are used in each year.)

1. Inflation by reciprocals of selection probabilities
There is one probability for each sampling stage: a) the probability of selecting the PSU; b) the probability of selecting the hospital; c) the probability of selecting the emergency service area (ESA) or OPD clinic from within the hospital; and d) the probability of selecting the visit within the ESA or clinic. The last probability is calculated to be the sample size from the ESA or clinic divided by the total number of visits occurring in that unit during that unit’s data collection period. The overall probability of selection is the product of the probabilities at each stage. The inverse of the overall selection probability is the basic inflation weight. Beginning with the 1997 data, the overall selection probabilities of some OPDs were permanently trimmed to prevent individual OPDs from contributing too much of their region’s total to OPD visit estimates.

2. Adjustment for nonresponse
NHAMCS data are adjusted to account for two types of nonresponse. The first type of nonresponse occurred when a sample hospital refused to provide information about its ED(s) and/or OPD(s), which were publically known to exist. In this case, the weights of visits to hospitals similar to the nonrespondent hospitals were inflated to account for visits represented by the nonrespondent hospitals where hospitals were judged to be similar if they were in the same region, ownership control group (government, non-Federal; voluntary nonprofit; or proprietary), and had the same metropolitan statistical area (MSA) status (that is, whether they were located in an MSA or not in an MSA). This adjustment was made separately by department type.

The second type of nonresponse occurred when a sample ESA or OPD clinic within a "respondent" hospital failed to provide completed Patient Record forms for a sample of its patient visits. The weights of visits to ESAs/OPD clinics similar to the nonrespondent ESAs/OPD clinics were inflated to account for visits represented by the nonrespondent ESAs/OPD clinics where ESAs/OPD clinics were judged to be similar if they were in the same region, ownership control group, MSA status group, and ESA/OPD clinic group. For this purpose, there were six OPD clinic groups: general medicine, pediatrics, surgery, OB/GYN, alcohol and/or substance abuse, and "other."

3. Ratio adjustments
Adjustments were made within hospital strata defined by region. Within the South and West, the adjustment strata for EDs were further defined by hospital ownership control groups. These adjustments were made separately for emergency and outpatient departments. For EDs, the adjustment was a multiplicative factor that had as its numerator the sum of annual visit volumes reported to EDs in sampling frame hospitals in the stratum and as its denominator the estimated number of those visits for that stratum. Through the 1996 NHAMCS, the adjustment for visits to OPDs was a multiplicative factor, which had as its numerator the number of OPDs reported in sampling frame hospitals in the stratum and as its denominator the estimated number of those OPDs for that stratum. The data for the numerator and denominator of both adjustments were based on figures recorded for the data year in the SMG Hospital Market Data Base.

Beginning with the 1997 NHAMCS, the adjustment for OPD estimates was replaced by a ratio that had as its numerator the weighted OPD visit volumes of hospitals in the full NHAMCS sample (16 hospital panels) and as its denominator the weighted OPD visit volumes of hospitals in the 13 hospital panels included in that year’s sample. This adjustment used visit volumes that were based on the most recent survey data collected from hospitals that had participated in NHAMCS for at least one year. For hospitals that had never participated, visit volumes were obtained by phone, from SMG data, or by using the average of visit volumes for refusal hospitals which had converted to respondent status in the 1998 survey.