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Volume 8, Number 5—May 2002


Sentinel Surveillance: A Reliable Way To Track Antibiotic Resistance in Communities?

Stephanie J. Schrag*, Elizabeth R. Zell*, Anne Schuchat*, and Cynthia G. Whitney*
Author affiliations: *Centers for Disease Control and Prevention, Atlanta, Georgia, USA

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Table 2

Number of isolates required to estimate accurately %PNSP in a given area and percentage of sentinel laboratory groups that met sample size requirements

Area Actual %PNSP (target range) No. of isolates needed to estimate %PNSPa % of sentinel groups of 5 laboratories with > no. of required isolates
CA 15 (10-20) 94 100
CT 18 (13-23) 172 3
GA 33 (28-38) 243 40
MD 22 (17-27) 183 12
MN 20 (15-25) 163 70
NY 15 (10-20) 97 100
OR 21 (16-26) 120 100
TN 35 (30-40) 191 0

a No. of isolates, n, required to estimate the area’s actual %PNSP (P) within 5 percentage points (d=0.05) with 95% confidence (Z=1.96) is: n= (Z2 P(1-P))/d2, where d is the range of accepted variation around the actual %PNSP, and Z is the Z-score range within which values must fall. Because the total no. of isolates per area, N, was small, we corrected this estimate for finite population size: n=n/[1+(n-1)/N]. There is no power associated with this estimate (14).
%PNSP, percent of penicillin-nonsusceptible pneumocooccal isolates.

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