A Complete Example
A Complete Example
- To see a complete linear correlation and regression analysis, in a practical setting, as a cohesive whole.
In the preceding sections numerous concepts were introduced and illustrated, but the analysis was broken into disjoint pieces by sections. In this section we will go through a complete example of the use of correlation and regression analysis of data from start to finish, touching on all the topics of this chapter in sequence.
In general educators are convinced that, all other factors being equal, class attendance has a significant bearing on course performance. To investigate the relationship between attendance and performance, an education researcher selects for study a multiple section introductory statistics course at a large university. Instructors in the course agree to keep an accurate record of attendance throughout one semester. At the end of the semester 26 students are selected a random. For each student in the sample two measurements are taken: , the number of days the student was absent, and , the student's score on the common final exam in the course. The data are summarized in Table 10.4 "Absence and Score Data".
Table 10.4 Absence and Score Data
A scatter plot of the data is given in Figure 10.13 "Plot of the Absence and Exam Score Pairs". There is a downward trend in the plot which indicates that on average students with more absences tend to do worse on the final examination.
Figure 10.13 Plot of the Absence and Exam Score Pairs
The trend observed in Figure 10.13 "Plot of the Absence and Exam Score Pairs" as well as the fairly constant width of the apparent band of points in the plot makes it reasonable to assume a relationship between and of the form
First we perform preliminary computations that will be needed later. The data are processed in Table 10.5 "Processed Absence and Score Data".
Table 10.5 Processed Absence and Score Data
Adding up the numbers in each column in Table 10.5 "Processed Absence and Score Data" gives
Rounding these numbers to two decimal places, the least squares regression line for these data is
The goodness of fit of this line to the scatter plot, the sum of its squared errors, is
This number is not particularly informative in itself, but we use it to compute the important statistic
The size and sign of the slope indicate that, for every class missed, students tend to score about 5.23 fewer points lower on the final exam on average. Similarly for every two classes missed students tend to score on average fewer points on the final exam, or about a letter grade worse on average.
Since 0 is in the range of x-values in the data set, the y-intercept also has meaning in this problem. It is an estimate of the average grade on the final exam of all students who have perfect attendance. The predicted average of such students is .
Before we use the regression equation further, or perform other analyses, it would be a good idea to examine the utility of the linear regression model. We can do this in two ways: 1) by computing the correlation coefficient to see how strongly the number of absences and the score on the final exam are correlated, and 2) by testing the null hypothesis (the slope of the population regression line is zero, so is not a good predictor of ) against the natural alternative (the slope of the population regression line is negative, so final exam scores go down as absences go up).
The correlation coefficient r is
Turning to the test of hypotheses, let us test at the commonly used 5% level of significance. The test is
From Figure 12.3 "Critical Values of ", with degrees of freedom , so the rejection region is . The value of the standardized test statistic is
which falls in the rejection region. We reject in favor of . The data provide sufficient evidence, at the 5% level of significance, to conclude that is negative, meaning that as the number of absences increases average score on the final exam decreases.
As already noted, the value gives a point estimate of how much one additional absence is reflected in the average score on the final exam. For each additional absence the average drops by about 5.23 points. We can widen this point estimate to a confidence interval for . At the 95% confidence level, from Figure 12.3 "Critical Values of " with degrees of freedom, . The 95% confidence interval for based on our sample data is
or (−7.38,−3.08). We are 95% confident that, among all students who ever take this course, for each additional class missed the average score on the final exam goes down by between 3.08 and 7.38 points.
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