Stat 1000: Tips for Web Assign HW 02
Published: Fri, 01/28/11
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Tips for Web Assign HW 02
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.
For the JMP 8 part of the assignment, here are some tips:
Once you have installed JMP 8 and opened it, you are shown a menu with various buttons to click. You will almost always click "New Data Table" to enter new data. In the rare event they have given you a JMP file with the data already entered in it, you will simply open that file which would probably already open JMP for you. If you happen to enter data in yourself and save the file (a good idea), you can select "Open Data Table" to open up the saved file.
To copy and paste data into JMP: First, of course, select and copy the given data set. Now, open JMP and click "New Data Table". In the toolbar at top select "Edit" then "Paste with Column Names". That pastes all the data in and names the column appropriately.
Note, you must save your document as a PDF file to upload it into Web Assign (no other format will be accepted). If you don't know how to do this for the software you are using try the help files or Google "save as pdf file" for some helpful steps or programs that enable you to save documents that way for free. MS Word 2007 is capable of saving as pdf. If you are using a different program and do not have a "save as pdf" option, download the "pdf writer" they give you in Web Assign or Stats Portal. If you are unable to "save as pdf", try selecting "print" as though you were going to print your file, then select your "pdf printer" rather than your physical printer in the print options. That will then save your file in pdf form.
If you are taking the course by distance/online (Section D01) click here to see your tips for HW 02.
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.
Study Lesson 1 in my study book (if you have it) to learn the concepts involved in HW 02.
Ignore any references to JMP 6SE or Crunchit!. You are using JMP 8 in this course. The assignment is just an old assignment that they forgot to update. Use JMP 8 anytime they tell you to use computer stuff.
Question 3 should be done manually. Note to enter the answers correct to 0.1, they mean round your answers off to one decimal place.
Question 4 should be done manually. Be sure to read Appendix D at the back of my book to learn how to use Stat Mode in your calculator to compute a mean and standard deviation quickly. By "nearest decimal place", they mean round your answers off to one decimal place.
Question 5 (the IQ and GPA question):
Click the link to the data file, then select and copy the entire data set (you can click "Ctrl A" on your keyboard to select all, then click "Ctrl C" to copy it all). Having opened a "New Data Table" in JMP, select "Edit" then "Paste with Column Names" to paste the data in. Double-click the "iq" column name at top and confirm that JMP has the "Data Type" as "Numeric" and the "Modeling Type" as "Continuous", changing those settings in the drop-down list if necessary. Click OK. Do the same for the "gpa" column. Important: Double-click the "gender" column and make sure that JMP has the "Data Type" as "Character" (it probably doesn't) and the "Modeling Type" as "Nominal" (it probably doesn't), changing those settings in the drop-down list if necessary. Click OK. Finally, take a look at the last row of data that has been pasted into JMP. If it just shows a bunch of dots instead of numbers, click that row to highlight it then right-click and select "delete rows" to delete that row. Of course, do not delete any row that has numbers (data) in it!
To find the mean, standard deviation and median in part (a):
Select "Analyze" then "Distribution". Highlight "iq" in the pop-up menu and click the "Y, Columns" button. Click OK. You are then taken to a screen that shows a histogram among other things. You will find the mean and standard deviation in the "Moments" section and the median in the "Quantiles" section.
To make the boxplots and histogram in part (b): In the toolbar at the top of your data spreadsheet, select "Analyze" then "Distribution". Select the "gpa" column and click the "Y, Columns" button. Click OK. Your histogram appears sideways but they didn't ask you to switch it horizontally, so don't bother. If they want to see it the typical way (and they will request that if they want it), click the red triangle next to your variable above the histogram and select Histogram Options from the drop-down menu. Select Horizontal Layout. Click the red triangle next to "gpa" and select "quantile boxplot" (if it isn't checked already) and "outlier boxplot" as well to get the desired boxplots. Click the blue triangles next to "Quantiles" and "Moments" to hide that stuff, then "select all" (click "Ctrl A" on your keyboard) and then "copy" (click Ctrl C). Paste it into your document. Be sure to type in your answers to the question they ask in part b underneath the graphs you pasted into your document. Remember how skewness and/or outliers affects a mean and median.
To make the side-by-side boxplots in part (c): Back in your data spreadsheet, select "Analyze" then "Fit Y By X". Highlight "gpa" and click "Y, Response". Highlight "gender" and click "X, Factor". Click OK. Now click the red triangle and select "Display Options", then select "Box Plots" to get your side-by-side boxplots. Select all and copy and paste into the same document you already have in part (b). Make sure you type your answer to their question below these boxplots in your document. You can now save the file and upload it into Web Assign.
Question 6 should be done manually. Read my section in Lesson 1 on "The Effect of Changing Units on Centre and Spread" to properly prepare for this question.