Microbiology A nine-laboratory cooperative study was performed to evaluate quality control for susceptibility tests with 30-µg netilmicin disks [6]. Each laboratory tested three standard control strains on a different lot of Mueller-Hinton agar, with 150 tests performed per laboratory. For protocol control, each laboratory also performed 15 additional tests on each of the control strains using the same lot of Mueller-Hinton agar across laboratories. The mean zone diameters for each of the nine laboratories are given in Table 6.11. *6.18 Provide a point and interval estimate (95% CI) for the mean zone diameter across laboratories for each type of control strain, assuming each laboratory uses different media to perform the susceptibility tests. *6.19 Answer Problem 6.18 assuming each laboratory uses a common medium to perform the susceptibility tests. *6.20 Provide a point and interval estimate (95% CI) for the interlaboratory standard deviation of mean zone diameters for each type of control strain, assuming each laboratory uses different media to perform the susceptibility tests. *6.21 Answer Problem 6.20 assuming each laboratory uses a common medium to perform the susceptibility tests. 6.22 Are there any advantages to using a common medium versus using different media for performing the susceptibil- ity tests with regard to standardization of results across laboratories?