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Biostatistics

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6.2 CONFIDENCE INTERVAL FOR A POPULATION MEAN 169<br />

Solution:<br />

Since the sample size is fairly large (greater than 30), and since the population<br />

standard deviation is known, we draw on the central limit theorem and<br />

assume the sampling distribution of x to be approximately normally distributed.<br />

From Appendix Table D we find the reliability coefficient corresponding<br />

to a confidence coefficient p of .90 to be about 1.645, if we interpolate. The<br />

standard error is s x ¼ 8=<br />

ffiffiffiffiffi<br />

35 ¼ 1:3522, so that our 90 percent confidence<br />

interval for m is<br />

17:2 1:645ð1:3522Þ<br />

17:2 2:2<br />

15:0; 19:4 &<br />

Frequently, when the sample is large enough for the application of the central limit<br />

theorem, the population variance is unknown. In that case we use the sample variance as a<br />

replacement for the unknown population variance in the formula for constructing a<br />

confidence interval for the population mean.<br />

Computer Analysis When confidence intervals are desired, a great deal of time<br />

can be saved if one uses a computer, which can be programmed to construct intervals from<br />

raw data.<br />

EXAMPLE 6.2.4<br />

The following are the activity values (micromoles per minute per gram of tissue) of a<br />

certain enzyme measured in normal gastric tissue of 35 patients with gastric carcinoma.<br />

.360 1.189 .614 .788 .273 2.464 .571<br />

1.827 .537 .374 .449 .262 .448 .971<br />

.372 .898 .411 .348 1.925 .550 .622<br />

.610 .319 .406 .413 .767 .385 .674<br />

.521 .603 .533 .662 1.177 .307 1.499<br />

We wish to use the MINITAB computer software package to construct a 95 percent confidence<br />

interval for the population mean. Suppose we know that the population variance is .36.<br />

It is not necessary to assume that the sampled population of values is normally distributed<br />

since the sample size is sufficiently large for application of the central limit theorem.<br />

Solution:<br />

We enter the data into Column 1 and proceed as shown in Figure 6.2.2 . These<br />

instructions tell the computer that the reliability factor is z, that a 95 percent<br />

confidence interval is desired, that the population standard deviation is .6, and<br />

that the data are in Column 1. The output tells us that the sample mean is .718,<br />

the<br />

p<br />

sample standard deviation is .511, and the standard error of the mean,<br />

s= ffiffi pffiffiffiffiffi<br />

n is :6= 35 ¼ :101. &<br />

We are 95 percent confident that the population mean is somewhere between .519<br />

and .917. Confidence intervals may be obtained through the use of many other software<br />

packages. Users of SAS ® , for example, may wish to use the output from PROC MEANS or<br />

PROC UNIVARIATE to construct confidence intervals.

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