Stat 1000: Tips for Web Assign HW 04
Published: Fri, 02/18/11
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General Tips for Web Assign and JMP
When working with Web Assign, always enter the answer to one specific box and then click "Submit Answer" to confirm that is correct before you move on to another box. Do not enter several answers all at once in several boxes before you click "Submit Answer". You risk being marked wrong due to some typo or something.
For some strange reason, JMP 8 occasionally computes wrong answers even if you have copied and pasted your data correctly. I suggest that, if it is feasible, type the given data into your calculator (in Stat mode as shown in Appendix D of my book), and have your calculator compute the sample mean. Compare that answer with JMP's answer for the sample mean. If they are the same, everything is fine. If they are not the same, close JMP 8 and restart it, recopy and paste the data, and check again. Sometimes you have to do this 2 or 3 times before JMP finally works. If it is not feasible to use your calculator to compute the sample mean, have JMP do the question 2 or 3 times, being sure to restart JMP and recopy the data each time, and confirm that JMP gives you the same answer each time before risking entering the results into Web Assign.
If you are taking the course by distance/online (Section D01) click here to see your tips for HW 04.
Study Lesson 2 in my study book (if you have it) to learn the concepts involved in HW 03.
Question 1 is similar to my question 2 in Lesson 2.
Questions 2 and 3 are just like my questions 5 and 6 in Lesson 2.
Question 4 is similar to my questions 3 and 4 in Lesson 2.
Question 5 is a good run through of all things X-bell curve. Be sure to have studied questions 9 to the end in my Lesson 2 before attempting it. There is a flaw in questions a and b though. Although they ask you for the "percent", Web Assign will mark you wrong if you enter the percent. Leave it as the proportion decimal value. For example, if you figured out the proportion is .1234, type in .1234, do not type in 12.34%.
Question 6:
For the JMP 8 part of the assignment, here are some tips:
Open a "New Data Table" in JMP. To copy and paste the data into JMP, in the toolbar at top select "Edit" then "Paste with Column Names". Double-click the "gpa" column heading and make sure the Data Type is Numeric and the Modeling Type is Continuous, using the drop-down menus to fix that if necessary. Double-click the "sex" column heading and make sure the Data Type is Character and the Modeling Type is Nominal, using the drop-down menus to fix that if necessary. Click OK.
To get side-by-side boxplots: In the toolbar at the top, select Analyze then select Fit Y By X. In the pop-up menu, highlight the gpa column and click the "Y, Response" button. Highlight the sex column and click the "X, Factor" button. Click OK. You will then see a graph with a vertical array of dots for the males (1) and the females (2). Click the red triangle next to "Oneway Analysis ..." and select "Display Options". You will then be able to select "Box Plots" in the Display Options sub-menu.
To get normal quantile plots: They want you to make a normal quantile plot for the males and a separate plot for the females. In the toolbar at the top, select "Analyze" , then "Distribution". In the pop-up menu, highlight "gpa" and click "Y, Column". Highlight "sex" and click "By". Click OK. You now see separate histograms for the males and for the females. Click the red triangle next to "gpa" for both the males and for the females, and select "Normal Quantile Plot" to get a separate normal quantile plot for the males and females.
A normal quantile plot checks to see if a sample's distribution appears to be normal. If the data follows a normal distribution, the normal quantile plot will look like a rising diagonal line. If the plot looks curved rather than linear, there is evidence the data is not normal. However, none of that seems to matter in your question, all they ask is which students are clear outliers, which, personally, I think is much easier to see from your side-by-side boxplots than from your normal quantile plots.
Study Lesson 3 in my book, if you have it, to prepare for this assignment.
Question 1:
To compute the correlation coefficient by hand, follow my example in Lesson 3, question 1, part (c). Note, you are not given the means and standard deviations for x and y already, so you are certainly allowed to use the Linear Regression Stat Mode on your calculator to tell you the means and standard deviations of both x and y. Put your calculator in Linear Regression Stat Mode (see Appendix D of my book). After you enter all the data, you can ask it for the mean and standard deviation of the x values and the mean and standard deviation of the y values. For example, Sharps use "RCL 4" to get x-bar and "RCL 7" to get y-bar. "RCL 5" gives you Sx and "RCL 8" gives you Sy.
Note, only those of you who have my blue or green study book have an example that follows exactly the headings given in this question. Even though they tell you to do everything to three decimal places, don't do that. Record every single decimal place your calculator gives you for each calculation, or else your answers won't be accurate enough. I suggest you do everything on paper first, then you can type in the results, rounding all of your numbers off to 3 decimal places at that time (even though you actually did the calculations using all the decimal places). Of course, your calculator actually tells you the value of r, so you can use that as a check.
Question 2 is just an algebra question. They give you three of x, y, a, and b and want you to figure out the missing one. Sub the givens into the appropriate places of
y = a + bx and solve what is missing.
Question 3 is a good run through of the formulas I show you in Lesson 3.
Question 4 uses JMP.
Here is how to use JMP for linear regression. First copy and paste the data into a New Data Table the usual way (see my previous homework tips if you are not sure how to paste the data). If you have to type the data in manually, simply double-click the space to the right of "Column 1" to create "Column 2". Enter the X data down column 1 and the Y data down column 2. Be sure to double-click each column to give it an appropriate name and to ensure the Data Type is Numeric and the Modeling Type is Continuous.
Select Analyze, then Fit Y By X. Highlight the column you have determined should be X, and click the X, Factor button. Highlight the column you have determined should be Y and click the Y, Response button. Click OK.
You should now see a scatterplot. Click the red triangle above the scatterplot and select Fit Line and JMP will draw in the least-squares regression line. Note, it shows you the regression equation directly below the scatterplot. JMP also shows you the value of r-squared (the coefficient of determination), rather than r, the correlation coefficient. Remember, the coefficient of determination is the percentage of y's variation explained by the regression equation. You can always square root this number to get r, the correlation coefficient, but use your scatterplot to help you decide if r is negative or positive because your calculator can't tell you that.
If you want to get rid of anything, click the red triangle and deselect anything you don't want to see. Note, if you click the blue triangle next to something, that will make part of the output disappear as well, if you wish. Just click the blue triangle again to make it reappear.