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Biostatistics

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9.5 USING THE REGRESSION EQUATION<br />

If the results of the evaluation of the sample regression equation indicate that there is a<br />

relationship between the two variables of interest, we can put the regression equation to<br />

practical use. There are two ways in which the equation can be used. It can be used to<br />

predict what value Y is likely to assume given a particular value of X. When the normality<br />

assumption of Section 9.2 is met, a prediction interval for this predicted value of Y may be<br />

constructed.<br />

We may also use the regression equation to estimate the mean of the subpopulation<br />

of Y values assumed to exist at any particular value of X. Again,ifthe<br />

assumption of normally distributed populations holds, a confidence interval for this<br />

parameter may be constructed. The predicted value of Y and the point estimate of the<br />

mean of the subpopulation of Y will be numerically equivalent for any particular value<br />

of X but, as we will see, the prediction interval will be wider than the confidence<br />

interval.<br />

Predicting Y for a Given X If it is known, or if we are willing to assume<br />

that the assumptions of Section 9.2 are met, and when s 2 yjx<br />

is unknown, then the 100 ð 1 aÞ<br />

percent prediction interval for Y is given by<br />

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />

u<br />

^y t ð1 a=2Þ<br />

s yjx 1 þ 1 n þ x p x 2<br />

t<br />

P<br />

ð xi x Þ 2<br />

9.5 USING THE REGRESSION EQUATION 441<br />

(9.5.1)<br />

where x p is the particular value of x at which we wish to obtain a prediction interval for Y<br />

and the degrees of freedom used in selecting t are n 2.<br />

Estimating the Mean of Y for a Given X The 100ð1 aÞ percent<br />

confidence interval for m yjx , when s 2 yjx<br />

is unknown, is given by<br />

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />

u1<br />

^y t ð1 a=2Þ<br />

s yjx<br />

n þ x p x 2<br />

t<br />

P<br />

ð xi x Þ 2<br />

(9.5.2)<br />

We use MINITAB to illustrate, for a specified value of X, the calculation of a 95 percent<br />

confidence interval for the mean of Yand a 95 percent prediction interval for an individual Y<br />

measurement.<br />

Suppose, for our present example, we wish to make predictions and estimates about<br />

AT for a waist circumference of 100 cm. In the regression dialog box click on “Options.”<br />

Enter 100 in the “Prediction interval for new observations” box. Click on “Confidence<br />

limits,” and click on “Prediction limits.”<br />

We obtain the following output:<br />

Fit Stdev.Fit 95.0% C.I. 95.0% P.I.<br />

129.90 3.69 (122.58, 137.23) (63.93, 195.87)

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