2006 HCSDB Adult Sampling Report - Tricare

2006 HEALTH CARE SURVEY OF DOD BENEFICIARIESChapter6DPSOH6L]HVThe total sample size for the QBS was determined based on the sample size appropriate for eachstratum. Because the strata are also important analytic domains, this strategy ensures thatsamples drawn from each stratum will be large enough to meet precision requirements. Inaddition, stratification with approximately optimum allocation to strata can be effective in reducingsampling errors of survey estimates. In this chapter, we present the procedures used for samplesize allocation for the QBS, including the requirements, response rates, and how the sample sizeswere finally determined.$ 35(&,6,215(48,5(0(176Precision requirements and expected response rates were the basis for determining stratum-levelsample sizes. These requirements were defined to ensure adequate precision for constructing 95percent confidence intervals. The QBS estimates the proportion of beneficiaries with certainattributes for particular domains of interest. When the sample size is large enough, we canassume that estimated proportions will follow approximate normal distributions according to theCentral Limit Theorem (Skinner, Holt, and Smith 1989). The resulting 100(1-α) percent confidenceinterval for a proportion of interest P is based on the standard formula:(IV.1)1−α2( )p ± z V p = p±HLz αwhere p is an estimate of P,1 − 2is the 100(1-α/2)th percentile point from the standard normaldistribution with a mean of zero and a standard deviation of one, V(p) is the variance of theestimate, and HL is the half-length of the two-sided 95 percent confidence interval, or0.975( )HL z V p= .For the QBS, precision requirements specified that the HL of the 95 percent confidence interval in(IV.1) for a given estimate should be less than or equal to a specified value. Because themaximum HL value occurs for P = 0.5, the precision requirements for the HLs were set for P valuesof 0.5. This helped to ensure that HLs for all estimates would be less than or equal to the specifiedvalues. Combining four quarters of the QBS should yield catchment-area-level estimates withprecision levels of 5 percentage points. However, as response is not constant among catchmentareas, final precision levels may vary among catchment areas. These precision requirements applyto the four noncatchment areas strata, as well.% 5(63216(5$7(6After calculating the number of eligible respondents needed to achieve the precision requirements,we inflated the resulting sample sizes to account for survey nonresponse. Average response ratesfrom the 2005 Adult HCSDB were used to approximate the expected QBS response rates.03/14/06 11