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Beetles and beavers Do beavers benefit beetles? Researchers laid out 23 circular plots, every 4 meters in diameter, in an area where beavers were cutting down cottonwood trees. In each plot, they counted the number of stumps from trees cut by beavers and the number of clusters of beetle larvae. Ecologists believe that the new sprouts from stumps are more tender than other cottonwood growth, so beetles prefer them. If so, more stumps should

produce more beetle larvae.31 Here is the computer output for regression of y = number of beetle larvae on x = number of stumps:

a. Calculate and interpret the residual for the plot that had 2 stumps and 30 beetle larvae.

b. Interpret the slope.

c. By how much does the actual number of larvae typically vary from the values predicted by the least-squares regression line with x = number of stumps?

d. What percent of the variability in a number of larvae is accounted for by the least-squares regression line with x = number of stumps?

Short Answer

Expert verified

Part (a) Residual is 7.498638

Part (b) The number of beetle larvae increases by 11.893733beetle larvae per stump.

Part (c) The actual number of beetle larvae divided by 6.419386 beetle larvae is the projected number of beetle larvae using the equation of least square regression line deviation.

Part (d) The least square regression line using the number of stumps as an explanatory variable explains 83.9144 percent of the variation in the number of beetle larvae.

Step by step solution

01

Part (a) Step 1: Given information

The relationship of number of larvae and the number of stumps is given in the question.

02

Part (a) Step 2: Concept

The formula used:Residual=y−yÁåœ

03

Part (a) Step 3: Calculation

The least square regression line's general equation is:

yÁåœ=b0+b1x

As a result, the estimate of the constant b0is given in the computer output's row "Intercept" and column "Estimate" as:

b0=−1.286104

In the computer output, the estimate of the slope b1is reported in the row "Number of stumps" and the column "Estimate" as:

b1=11.893733

As a result, when we plug the values into the general equation, we get:

yÁåœ=b0+b1x⇒yÁåœ=−1.286104+11.893733x

As a result, the number of beetle larvae with two stumps was as follows:

yÁåœ=−1.286104+11.893733x=−1.286104+11.893733(2)=22.501362

Thus the residual will be calculated as:

Residual=y−yÁåœ=30−22.501362=7.498638

This means that when using the regression line to forecast the number of larvae when there are two stumps, we underestimated the number of larvae by 7.498638beetle larvae.

04

Part (b) Step 1: Calculation

The question specifies the link between the quantity of larvae and the number of stumps. The regression line is as follows:

yÁåœ=−1.286104+11.893733x

The slope is the coefficient of x in the least-squares regression equation, and it reflects the average rise or decrease of y per unit of x as we all know. Thus,

b1=11.893733

As a result, the average number of beetle larvae per stump increases by 11.893733 beetle larvae.

05

Part (c) Step 1: Calculation

The question specifies the link between the quantity of larvae and the number of stumps. The regression line is as follows:

yÁåœ=−1.286104+11.893733x

The standard error of the estimate s is reported as follows in the computer output after "Root Mean square error":

s=6.419386

The standard error of the estimations, as we all know, is the average error of forecasts, and thus the average difference between actual and predicted values. As a result, the anticipated number of beetle larvae using the equation of least square regression line differed from the actual number of beetle larvae by an average of 6.419386 insect larvae.

06

Part (d) Step 1: Calculation

The question specifies the link between the number of larvae and the number of stumps. The regression line is as follows:

yÁåœ=−1.286104+11.893733x

The coefficient of determination is given as follows in the computer output after "RSquare":

r2=0.839144=83.9144%

The coefficient of determination, as we know, is a measurement of how much variation in the answers y variable is explained by the least square regression model with the explanatory variable. As a result, the least square regression line utilizing the number of stumps as an explanatory variable can explain 83.9144 percent of the variation in the number of beetle larvae.

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