Stat 2000: REVISED Tips for Assignment 3

Published: Wed, 03/16/16

There was a typo in my tips for question 6 below that caused some confusion.
Final Exam Prep Seminar April 12
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Tips for Assignment 3
Please note that I made major changes to my book in September 2014.  If you are using a book older than September 2014, you are missing about 100 pages of new material and an entirely new lesson on Probability.

Study Lesson 6: Introduction to Probability and Lesson 7: Discrete Probability Distributions in my book (if you have it) to prepare for this assignment 

To type in formulas you are using and to show your numbers subbed into the formulas you can click the Equation Editor button in the toolbar that looks like the Sigma Summation symbol (you have to click the "..." other options button to see the sigma formula input button.  Then click the various buttons to make your fractions and enter the symbols.  However, the Equation Editor is extremely slow and clunky.  Personally, I would never use it.  Just type ordinary text explaining what you are doing if you think you should show some work.

Exception: Always do any JMP stuff open-book.  Have my tips in front of you, and let me guide you step-by-step through any JMP stuff.  JMP is just "busy" work.  The sooner you get it done and can move on to productive things like understanding the concepts and interpreting the JMP outputs, the better off you will be.  Then again, since you never have to upload the JMP printouts, perhaps you might not even bother to do the JMP at all.  Most questions can be answered by hand even when they told you to use JMP.
Question 1
I show you how to determine a Sample Space through the use of two-way tables if necessary in Lesson 6 of my book.  Note that all you are asked for is the sample space in each part, so your answer would be something like this (don't forget to use those squiggly brackets "{}").For example, here is the sample space for the outcome of flipping a coin twice where H=heads and T=tails: 
{HH, HT, TH, HH}.

Don't state the probabilities!  You are not asked for the probabilities.  In fact, in many of the situations students will be given for this problem, it is impossible to know the probabilities.  You are just asked for the sample space.

You may find a tree diagram helpful for part (c).  Note that the outcomes will vary in length.  If you get G right away, you stop, but if you get S or C, you select again.  Remember, this is without replacement, so there will be a limited number of trials until the gold coin is selected.  Sample space would look something like this: {G, SG, CG, ...}

Part (d) is similar to part (c) but will be longer.
Question 2
Although this question appears to be a 3-circle Venn diagram problem (similar to my Lesson 6, question 18), they have left key information out that renders it not worth the effort to try to make the diagram right away.  You are better off just taking each question as it comes and answering it directly.

Part (a)
Note that every entry in a Venn diagram represents an outcome.  For example, a typical 2-circle Venn has 4 entries (the number where the two circles intersect), the number in A but outside of B, the number in B but outside of A, and the number in the rectangle but outside of both A and B).  Since there are 4 entries, there are 4 outcomes in the sample space.

Here, even though we don't have enough information to fill in the Venn diagram easily, we can still picture this problem as a 3-circle Venn diagram.  Count the number of distinct regions inside the rectangle of a 3-circle Venn to identify the number of outcomes in the sample space. 

Hint: There are 8 outcomes in the sample space.  Each region in the Venn can be uniquely identified in terms of each of the three types of fans.  For example, the section that is inside the J circle, but outside of the B and G circles would be described as JB^cG^c where, by B^c I mean B with a c superscript to denote the complement of B, and similarly G^c is G with a c superscript to represent the complement of G.

Typing Tip:  Since the text boxes you have to type your answers in have the same difficulty showing the c superscript to denote a complement, you may find it convenient to use a different symbol to denote the complement.  I suggest the single quote or prime symbol '.  Make sure you define your symbols.  You could say:
  • Let J' = the complement of J
  • Let B' =  the complement of B
  • Let G' = the complement of G
Now, it will be easier to type in the sample space.  For example, the section in the Venn that represents people who follow the Jets, but do not follow the Bombers or the Goldeyes would be denoted JB'G' in your sample space.

You can also use two-way tables to identify all the outcomes in the sample space (but those two-way tables cannot reliably solve the probability because we cannot use the independence multiplication rule).  Along the top list Jets fans (two columns: either J, you are a Jets fan, or J-complement, you are not).  Down the side, list Bombers fans (B or B-complement).  Then bring in the Goldeyes fans in a second table.

Part (b)
This problem is easily solved using the General Addition Rule formula for P(A or B) as given in Lesson 6 of my book.  Just focus on Jets and Bombers.

P(A or B) = P(A) + P(B) - P(A and B)

Part (c)
This too can be solved using a little algebra and that General Addition Rule formula.  Hint: Focus on Jets and Goldeyes.

Part (d)
Keep in mind things you have learned from part (c), again you can use the General Addition Rule formula to solve this algebraically.  Focus on Bombers and Goldeyes.

Part (e)
Look at my similar question 16(d).  For example, you are given P(J), Jets, and P(B), Bombers, and you have also been given P(J and B), Jets and Bombers.  If these two are independent, P(J)*P(B) should match the given value for P(J and B).  If it matches, they are independent.  If it does not match, they are dependent.

After having answered parts (b) to (d), you know all the things to check independence of each pair of sports fans.

Part (f)
At this point, you now should know the probabilities P(J), P(B), P(G), P(J and B), P(J and G), P(B and G), and P(J and B and G).  So, take a moment to now fill in the entire 3-circle Venn diagram to assist you in answering the remaining questions.

Recall, if A and B are disjoint (or mutally exclusive), the P(A and B) = 0.  You already know what the relevant probabilities are to easily answer this question.  Also, if any of these pairs are disjoint, their circles on the Venn diagram should not intersect at all.

Part (g)
This is conditional probability!  They want P(J|G).  Use the formula.  I know, they say you can do it without calculations and that is true (because of the work you did checking independence back in part (e)), but who cares?  The formula should give you insight into why there actually was no need for calculation.

Part (h)
Just find the relevant regions in the Venn diagram and add them up.

Part (i)
This is conditional probability!  They want P(J|B^c).  Use the formula and your Venn diagram to assist.

Part (j)
This is conditional probability!  They want P(G|J and B).  Use the formula and your Venn diagram to assist.

Part (k)
Again, this is easily answered from your Venn.
Question 3
You can use a Two-way table to list all the outcomes in your sample space first, but, be careful!  You are sampling without replacement.  That will affect the probabilities.  The probability of the second coin depends on what was chosen first.  For example, P(GG) = 7/25 * 6/24 = 0.07.  Work out the complete probability distribution perhaps to assist you in answering all the questions.

My Lesson 6, questions 4 and 13 may be of some help in understanding how to do parts (a), (b) and (e).  Do note that (e) is conditional probability.

Note, the expected value is the mean value.  I show you how to compute mean and variance of X in Lesson 6, question 1 and I also do it more thoroughly in Lesson 7, questions 1 to 3.
Question 4
You have two normal distributions, J (javelin distance) and H (hammer distance).  You can use the properties of mean and variance I teach at the start of Lesson 4 to work out the mean and variance of J + H.  Since J and H are both normal distributions, J + H is also normal.  You can also compute the mean and variance for J - H.  You also know that J - H is normal.

Of course, to change your score into a z-score, you will have to use the standardizing formula you first learned in Stat 1000, z = (x - mu)/sigma or z = (x - μ)/σ.

Part (a)
We want J < H.  Rewrite that as J - H <  0.  So, let X = J - H using the mean and variance for J = H using properties of mean and variance, and find when X<0 on the X bell curve.

Part (b)
Let X = J + H and find the appropriate area on the X bell curve using the mean and variance for J + H.

Part (c)
We want J = H.  Rewrite that as X = J - H = 0.  Think carefully! What are you shading on your bell curve?
Question 5
I cover the Binomial Distribution in Lesson 7.  If you are ever asked to decide if a particular situation is binomial or not, remember, to be binomial, four conditions must be satisfied:
  1. There must be a fixed number of trials, n.
  2. Each trial can have only two possible outcomes, success or failure, and the probability of success on each trial must have a constant value, p.
  3. Each trial must be independent.
  4. X, the number of successes, is a discrete random variable where X = 0, 1, 2, ... n.
Hints:
  • If you are reading off numbers from a randomly selected row in the random number table, note that every row has 40 digits.  That is like 40 trials looking for whatever digit you may be looking for.  What is the probability that, at any moment on the table, the next digit is a 0, or a 1, or a 2, etc..
  • If you are selecting objects, are you sampling with replacement (independent trials) or without replacement (dependent trials)?
  • If you are given a Normal population, but are selecting a sample of size n, and want to see how many of them are greater than 62 (for example), THAT IS A BINOMIAL DISTRIBUTION!  You can use Table A to find what proportion are greater than 62.  That is your p.  Each trial, the person/thing either is greater than 62, or they are not.  And the chance they are greater than 62 is p.
  • If you are ever conducting trials until you get a desired result, that will never be binomial because you do not have a fixed number of trials, n.
  • If you are being dealt a hand of cards, that is definitely sampling without replacement.  How can you have a hand of 5 cards, for example, if you are replacing each card as it is dealt?
Question 6
I introduce the formula for mean and standard deviation of a binomial distribution in my Lesson 7, question 10.  Be careful that you are using the correct n and p for each question since they keep switching which flight and how many years and flights they are talking about.  Note that n is the total number of flights he has taken to a specific destination during the time period, not just the number that arrive on time.  I think they want you to understand that he is making 6 trips each year, and on each trip he has to take 3 flights.  So there are 6 first flights, 6 second flights and 6 third flights each year.

Therefore, if the question is talking about Flight 1 for one year, then n=6.  Similarly, for Flight 2 or Flight 3.  However, if the question is talking about 2 years, then n is doubled; 3 years would triple n.
Question 7
Standard Poisson stuff as taught in Lesson 7.  Make sure you are using the correct value for lambda.

Part (c) is really saying that there are 0 trees within a 1.5 metre radius.  What would be the average number of trees in that radius?  They don't tell you how many decimal places to use, so I would compute the average to 4 decimal places, our default number of decimals.
Question 8
Standard Poisson stuff as taught in Lesson 7.  Make sure you are using the correct value for lambda.