[janemorales]

I am having trouble with these probabilities. Did I do them correctly?

2. A survey of undergraduate students in the School of Business at Northern University revealed the following regarding the gender and majors of the students:
Major
Gender Accounting Management Finance total
Male 100 150 50 300
Female 100 50 50 200
total 200 200 100 500

a. What is the probability of selecting a female student? 40 or 40%
b. What is the probability of selecting a finance or accounting major? 6 or 60%
c. What is the probability of selecting a female or an accounting major? Which rule of
addition did you apply? 4 or 40% conditional
d. Are gender and major independent? Why? Yes because they are conditional to each other
e. What is the probability of selecting an accounting major, given that the person selected is a male? 2 or 20%
f. Suppose two students are selected randomly to attend a lunch with the president of the
university. What is the probability that both of those selected are accounting majors?

.2 or 20%

[morgaine300]

a & b are correct

c isn't a conditional. I don't know what they mean by "which" rule of addition. I only know one. A + B - (A intersection B). Number of females = 200. Number of accounting majors = 200. But this duplicates the 100 that is the intersection of the two, so you have to subtract it. That's the addition rule for figuring a union, which is what you're doing.

Did you learn to set up a joint probability table for these?

Unfortunately, the rest are incorrect and I don't think you understand conditionals. But that's nothing unusual. It would help a whole lot to see your work to know where you are going wrong.

d) The answer "they are conditional to each other" doesn't mean anything to me. Maybe you know what you're saying, but I don't. It sounds as though you're saying that because you can make them conditional to each other, that makes them independent. (And your answer isn't correct, so you're applying it wrong at any rate.)

A condition means you're eliminating down to one group. If that changes the probability of something, they are dependent. i.e. the probability of being male is .60. If you know that the person is an accounting major, does that change the probability they are male? If so, they are dependent. If I use the condition of accounting major and male stays at .60, then the condition had no effect on the probability of being male, and they are therefore independent of each other. Remember that independent means not affecting each other. A second roll of a die is independent, cause what came up on the first roll won't affect it. But does knowing someone is a management major change the probability of the person being male or female?

An example I'll never forget from my class. Instructor said a guest speaker would be coming in 20 minutes and asked what we thought the probability was of the person being over 6 feet tall. Answers aren't relevant. Then she said, what if I tell you the person is a pro basketball player. Now what do you think is the probability the person is over 6 feet tall? It's now not just anyone walking in; we've eliminated down to just pro basketball players, and of course the probability of the height will change. That is dependent.

You set up an equation like: P(A) = P (A given B). You can test whichever ones you want. If they actually equal each other, then they're independent. i.e. the probability of A didn't change when we eliminate down to just B. If they don't equal, they are dependent.

(e) Of course, you can't do the above until you learn how to do a conditional. I don't know where you got .20. I tested the most common mistake and that isn't it. The most common mistake would be to divide the 100 intersection of male and acct, by the 200 accounting majors. But that isn't what you did. It would be nice to know how you got the .20. To do a conditional, eliminate it down to the condition first. The condition is being male. There are 300 males. You've eliminated the females -- they don't count so you can't include them. Now that you're down to 300 males, how many are accounting majors? Out of those 300 males, what is now the probability of him being an accounting major?

f) OK, well, I've had a brain fart and this one has gone completely out of my head. But it isn't .20. (.16? Or am I over-simplifying here?)

[Unknown008]

For (d), according to your explanation, morgaine, I think that they are dependent (e.g. the probability of having a male in accounting is 50% instead of 60% for the total number of males.

As for the conditional thing, I know nothing but I do know about dependent and independent terms.

Now (e) I think that I made the same mistake as jane. I did not read the question properly lol and did the number of male accountants over the total number of students. But after rechecking, I found that it was 1/3 instead.

For (f) I got 0.159519... after rounding off, 0.16.

[morgaine300]

d -- yup, and some of the other tests changed it even more (I think one went to 75%)

e -- I don't think it's about reading improperly, though that could be what you did. I see people mess this up all the time because they do it backwards. This is where a piece of paper and colored pencils come in handy. :-) And yes it's 1/3.

f -- well, so we both got the same thing, but I'd prefer having some confirmation from someone else. I don't know why the probability of getting "2 of something" or whatever like that makes me nuts, but it does. (Unless it's binomial, or something I can just "see.")

[Unknown008]

Well, i got (f) by having