Stat 1000: Tips for Web Assign HW 11
Published: Wed, 03/30/11
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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 11.
There is no HW11 for your course.
Continue to study Lessons 8 and 9 in my book, if you have it, to prepare for this topic.
First, be sure to note
whether a question gives you σ, the population standard deviation, or s, the
sample standard deviation. That dictates whether you will use z or t when
making your confidence interval or testing your hypothesis. I would assume, at this stage, you are not likely
to be given σ most of the time.
Question 1 is standard confidence interval stuff using t* as I show in Lesson 8.
In question 2, you
are asked for the standard error of the sample mean, that is not the standard deviation. I discuss the Standard Error of the sample mean in
Lesson 8 of my book.
Note the margin of error of
any confidence interval is everything that comes after the +/- in the
appropriate formula.
Question 3
Look at my examples 8, 9 and 10 in
Lesson 9 to understand how to put bounds on a P-value if you are using t. BE
CAREFUL! In Web Assign, when you are entering the critical values and tail
area bounds into your boxes always put the smaller value in the left box
and the larger value in the right box because the < signs demand
that.
Question 4
This is a matched pairs
problem. Study my questions 19 and 20 in Lesson 9 to understand how to do
hypothesis tests for matched pairs.
To use JMP to do a matched
pairs test, copy and paste the data into New Data Table the usual way
then select "Analyze", "Matched Pairs". Highlight both columns of your matched
pairs data and click "Y, Paired Response" and click OK. You will then be given
all the numbers you need.
Be careful to note which way JMP is subtracting the pairs (it tells you in its printout). For example, if JMP says it is computing A - B, but you are specifically told to compute B - A, you can simply change the sign of your test statistic to fix that. Which is to say, if A - B produces a test statistic of t = -2.3, then B - A would produce a test statistic of t = +2.3. Similarly, the value for x-bar, the mean of the differences, would be the same but have the opposite sign. The sign of the standard deviation of the differences never changes. Similarly, this affects the signs of the lower and upper limits of the confidence interval for the differences that JMP makes. If JMP gave you the confidence interval for A - B, the confidence interval for B - A would be the same numbers but the opposite signs. Also note that changing the signs also changes the order of the limits. Which is to say, if the Upper Limit used to be +5, change the sign to -5 and that will now be the Lower Limit. Similarly, if the Lower Limit used to be +2, change the sign to -2 and that will now be the Upper Limit.
If you want to use Crunchit! (and I wouldn't) copy the "http:..." stuff for the
link to the data (shown at the top in your web browser) then select "Data" and
"Load Data from URL". Paste the link into the box and make sure the box saying
"use first line as column names" is selected. Make sure you have selected
"whitespace" as the delimiter to separate your columns. Now click "Statistics",
"T tests", and "Paired". Highlight column 1 in the first box and column 2 in
the second box. Be sure to select the correct alternative hypothesis and set
the appropriate confidence level, then click OK to get your
printout.
After using JMP or Crunchit! to do the problem, I also suggest you do it by hand since a question like this is always a possible exam question. However, by hand, the best you can do is put bounds on the P-value, whereas Web Assign requires an exact P-value.
Question 5
To use JMP: Select and copy the data and paste it into JMP the usual way. Note that the question only wants you to examine the red flowers. Therefore, be sure you only select and copy the data for the red flowers. That will also mean you will need to type in the names for the columns yourself at the top. Alternatively, you could select and copy all the data, then delete the rows in the JMP spreadsheet that do not have red flowers.
Select "Analyze, Distribution" and make "length" the "Y Column " and click OK to get the Histogram and stuff. Click the red triangle next
to the variable and select "Test Mean" from the drop-down list. Enter in the
mean from your null hypothesis. Click "OK" and JMP gives you the hypothesis test at the bottom of the printout. Look
at my questions 13 and 14 for examples of how to read this printout.
If you want to use Crunchit! to do a
hypothesis test using t, copy the "http:..." stuff for the link to the
data (shown at the top in your web browser) then select "Data" and "Load Data
from URL". Paste the link into the box and make sure the box saying "use first
line as column names" is selected. Now, you should see the data in your
spreadsheet. Select "Graphics" and then select "Histogram". Highlight the
appropriate column of data and click "OK" to get a histogram. Copy and paste it
into your document. Select "Statistics", then select "T tests", then select
"One-sample". In the pop-up window, make sure you click the appropriate
variable in the "Variable" box to highlight it. Type in the mean you are using
in your null hypothesis. Be sure you click whether you are doing a two-sided,
upper-tailed (greater than) or lower-tailed test (less than) or else you will
not be given the correct P-value. Click "OK". You now get a printout showing,
among other things, the test statistic and P-value.
In either case (JMP or Crunchit!), you can now select, copy
and paste your output to a file ready for upload as usual.
When they say "show all your steps", that does not mean you have to show how the calculations are being done, just go through the 5 steps and list what you have come up with for each step, using JMP or Crunchit!'s numbers wherever appropriate.
Make sure you read my section on P-values to learn how to
properly interpret your P-value. All that is required is one or two sentences.
Do not think you have to write an essay, and don't think you have to really
write something that a "layman" will understand. A layman is never going to
understand a P-value, even if you spent three pages trying to explain
it.
In either case (JMP or Crunchit!), you can now select, copy and paste your
output to a file ready for upload as usual.
Question 6 is just more practise at the concepts in Lessons 8 and 9.