For more information, please see full course syllabus of Probability

For more information, please see full course syllabus of Probability

### Hypergeometric Distribution

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

- Intro
- Hypergeometric Distribution
- Formula for the Hypergeometric Distribution
- Key Properties of Hypergeometric
- Example I: Students Committee
- Example II: Expected Number of Women on the Committee in Example I
- Example III: Pairs of Shoes
- Example IV: What is the Expected Number of Left Shoes in Example III?
- Example V: Using Indicator Variables & Linearity of Expectation

- Intro 0:00
- Hypergeometric Distribution 0:11
- Hypergeometric Distribution: Definition
- Random Variable
- Formula for the Hypergeometric Distribution 1:50
- Fixed Parameters
- Formula for the Hypergeometric Distribution
- Key Properties of Hypergeometric 6:14
- Mean
- Variance
- Standard Deviation
- Example I: Students Committee 7:30
- Example II: Expected Number of Women on the Committee in Example I 11:08
- Example III: Pairs of Shoes 13:49
- Example IV: What is the Expected Number of Left Shoes in Example III? 20:46
- Example V: Using Indicator Variables & Linearity of Expectation 25:40

### Introduction to Probability Online Course

### Transcription: Hypergeometric Distribution

*Hi and welcome back to the probability lectures here on www.educator.com, my name is Will Murray.*0000

*Today, we are going to be discussing the glamorously named hyper geometric distribution.*0006

*Let me tell you about the situation where you would use the hyper geometric distribution.*0011

*I set it up in terms of picking a committee of women and men.*0016

*The idea is that you have a larger group, you have a big group of N people.*0021

*There is N and there is a n in the hyper geometric distribution.*0029

*Make sure you do not get those mixed up.*0033

*You got N people total, all women, and N - R man.*0035

*What you are going to do is you are going to form a committee from this larger group.*0043

*Your committee is going to have n, that is the number of men and women you are going to put on your committee.*0048

*We want to emphasize here that this is an unordered choice.*0058

*You are going to just grab a group of people, it does not matter which order you are grabbing them in.*0061

*You are not going to have a chair of the committee, you are not going to have any special positions.*0066

*You are going to have a group of people, you can think of it may be as a team, a sports team.*0071

*It is without replacement meaning you cannot pick the same person twice.*0077

*You grab this group of people and then the question is, how many women did you end up with on your committee, out of all the possible men and women?*0081

*More specifically, what are the chances of getting exactly y women on your committee?*0092

*Our random variable here represents the number of women that you end up with on our committee.*0100

*Let us go ahead and look at all the parameters, there is a lot of them, and let us figure out the formula.*0106

*There is a lot of parameters here N is the total number of people that you are looking at.*0113

*That is the number people that are available to be selected on your committee.*0119

*R is the number of women available and that means that all that remain are men, that is N - R is the number of men available.*0125

*And then n is the number of people we are going to pick.*0135

*When I look at this large pool, let me draw a little Venn diagram here.*0140

*This large pool of N people available, there is N people available /all.*0145

*All of them are women, N - R of them are men.*0152

*We are going to create our committee of N people and that means that,*0157

*we want to find the probability of why those people being women which means that n - Y of those people are men.*0163

*The probability distribution formula looks very complicated but *0173

*I'm going to try to persuade you that it is actually a very easy formula to remember,*0178

*if you can remember this situation that we are describing.*0182

*The probability formula is R choose Y/N - R choose n – Y.*0187

*Multiply by that and then N choose n.*0194

*I want to emphasize that these are all binomial coefficients.*0199

*These are combinations, you will use the factorial formula to simplify these.*0202

*That looks like a very difficult formula to remember but it is not, and here is why.*0208

*The denominator that just represents, remember there is N people total and you are choosing n of them.*0214

*This is the total number of ways to choose your committee.*0221

*There is N people total and you are choosing n of those people to be on your committee.*0234

*If you are going to disregard gender, you are just making a choice of n people out of the total number of people.*0243

*Suppose you take gender into account and suppose you want to get exactly Y women on your committee.*0250

*You have a fixed number of women that you want to get on your committee.*0256

*Then you will look at all the women in the room and you would choose exactly Y of them to be on your committee.*0260

*There are women and you are choosing Y of them to be on your committee.*0266

*You are making a choice of Y people out of R women available.*0274

*Then, after you have chosen your women, you look around at all the men and you choose the number of men you need.*0280

*How many men do you need?*0286

*If you want to get Y women, that means you need n – Y men and how many men are available.*0288

*We said there is N – R, the number of men available.*0299

*This term really represents you choosing the men to be on your committee.*0303

*You have a certain number of ways you can pick the women.*0310

*You can have a certain number of ways you can pick the men.*0313

*You multiply those together, that gives you the total number of ways to pick your committee that has exactly Y women.*0316

*And then, you divide that by the total number of ways to pick your committee, if you do not pay any attention to gender at all.*0325

*That is actually, I think that is a fairly easy formula to remember, even though it looks very complicated.*0332

*It is definitely one of the most complicated probability distribution formula.*0339

*This Y here, the range for Y, you could have as few as 0 people, 0 women on your committee.*0344

*Or it is a n bit complicated here because the most number of women you can have on your committee would be N, *0350

*because that is the size of the committee, or R because that is the number of women available.*0359

*Whichever one of those is smaller, that is the maximum possible number of women you can have on your committee.*0364

*We need to get a couple of properties down with the hyper geometric distribution.*0373

*The most useful one is the mean, which you remember is the same as expected value.*0377

*The expected value of the hyper geometric distribution, this n × R/N.*0384

*N is the size of your committee, R is the number of women available, *0392

*and N is the total number of people in the room that you are choosing from.*0398

*The variance is really a kind of a nasty formula, I do not recommend memorizing it.*0403

*I do not use it very often but I wanted to record it for posterity, in case you do need it.*0409

*These are actual fractions, let me emphasize, these are not binomial coefficients.*0415

*This is just what it turns out to be.*0422

*Like I said, I do not really think there is a lot of intuition to be gained from this variance.*0424

*I do not think it is worth memorizing that formula.*0434

*The standard deviation, of course, is just the square root of the variance.*0436

*It is always the square root of the variance.*0440

*I just took the variance formula and took the square root of it, to get the standard deviation.*0442

*Let us go ahead and jump into some examples here.*0449

*In example 1, we got 33 students in a class and 12 women and 21 men.*0452

*We are going to pick a committee, maybe we are going to do a group project and 7 students are going in a group project.*0459

*I will pick 7 students at random, what is the chance that we will get exactly 5 women working on that project?*0465

*This is a hyper geometric distribution, let me set up the parameters here.*0471

*N is the total number of people available, that is 33.*0475

*R is the number of women in the room, that is 12.*0482

*That means that N - R is the number of men available, that is 21.*0486

*The number people on our committee is 7 and we are interested in the chance that we are going to end up with Y,*0499

*with 5 women on our committee, that is the value of Y or Y is 5.*0507

*That is because we want our committee to have exactly 5 women.*0512

*Let me write down the formula for the hyper geometric distribution.*0516

*P of Y is R choose Y, that is where we picked the women, × N -R men available, n - - Y men on our committee ÷ N ÷ n,*0520

*that is the total number of ways we could have chosen this committee or this group of students do a project.*0536

*I will just drop the numbers in.*0541

*R is 12, Y is 5, N - R is 21, n -y is 7 -5 is 2, N is 33, and n is 7.*0543

*I'm going to leave that as a fraction like that, I did not bother to work it out to a decimal.*0565

*It would be a fairly small number, if you actually worked out the numbers, it should be pretty small.*0572

*But it would be a load of factorials that I just did not want to calculate.*0577

*I did not think it would be very illuminating but it would be pretty small,*0582

*because if you pick 7 people at random from a class like this,*0587

*the chance you getting 5 women is very low because there is there is more men than women in this class.*0590

*Let me recap where those came.*0597

*First, I set up all my parameters, the N, R, n, n – R, and Y.*0601

*Then I just use the probability distribution formula for the hyper geometric distribution.*0607

*This is the formula, I know it looks difficult to remember but if you kind of think about what each one of those factors represents, *0612

*it is really not hard to remember the formula.*0619

*I think this formula kind of makes intuitive sense, if you think about the R choose Y*0625

*means you are picking Y women from R available women.*0631

*N -R being is the number of men available and n - Y is the number of men you want.*0637

*We multiply those together and N choose n is the number ways of choosing your committee in the first place.*0644

*We drop the numbers in for each one of those and we just give that as our answer.*0652

*That is our chance that the committee will contain exactly 5 women.*0656

*We are going to hang onto these numbers for the next example.*0660

*Remember the basic setup of this example and we will go ahead and take a look at that.*0663

*Example 2 was referring back to example 1.*0670

*In example 1, we were picking students from a class and we are picking a committee of 7 students, maybe a group project in a class.*0673

*Let me just remind you of the parameters from example 1.*0683

*We had N was the number of students in the class, 33.*0686

*R was the number women in the class, I got this from example 1, they were 12 woman in the class.*0692

*N was the number of people that we are picking to be on our committees, that is 7.*0698

*The expected number of women is the expected number of our random variable Y.*0706

*Y is the number of women on our committee.*0712

*We have a formula for the expected value of a hyper geometric random variable, the mean.*0723

*E of Y is n × r/N.*0734

*In this case, that n is 7 × r is 12, N is 33.*0740

*I guess we could simplify that, 12 and 33, you can take out a 3 from each of those.*0751

*7 × 4/11, that is a 28/11.*0757

*Our units here are women, that is the total number of women we expect on our committee.*0763

*Obviously, you cannot have fractions of women but on average, if we did this many times,*0769

*we would expect to see on average, 28/11 is a lot less than 3.*0775

*A little less than 3 women on the committee, on average.*0782

*To recap here, I got these parameters from example 1.*0788

*Example 1 setup how many people there were in the room, how many women, how many men,*0793

*how many people we are picking on our committee.*0797

*I got this formula for the mean from the third slide at the beginning of the lecture.*0800

*If you scroll back a couple of slides, you will see this mean formula.*0806

*I will just drop the numbers in and I simplified that down to a certain number of women.*0809

*Of course, in real life, we will either have 1 woman, or 2 women, or 3 women.*0816

*On average, we will have a bit fewer than 3 women on our committee.*0823

*In example 3 here, you open up your shoe closet and you do a shoe inventory.*0831

*It looks like you have 10 pairs of shoes in your closet.*0837

*You have lots of pairs of shoes in your closet.*0840

*You are getting ready to move to a new apartment.*0843

*You are in a hurry, you grab the nearest box you see and you start throwing your shoes in.*0846

*You are not really keeping track of which shoe matches up which.*0852

*You are just throwing them all in, you will unpack them after you move.*0855

*You start throwing your shoes in and you get 13 shoes in the box, and it is full.*0859

*You seal up the box and then you start to wonder, how many left shoes are in the box and how many right shoes are in the box?*0866

*In particular, what is the probability that there are exactly 5 left shoes and 8 right shoes in the box?*0873

*This is a hyper geometric distribution because if you think about it, it is just like selecting women and men to be on a committee.*0879

*You had a certain number of left shoes in your closet.*0890

*You have a certain number right shoes in your closet.*0893

*You grab some and put them in the box, it is just like selecting women and men to be on your committee.*0894

*Let me set up the parameters here for the hyper geometric distribution.*0900

*N is the total number of people in a room, or in this case, it is the total number of shoes in the closet,*0905

*before you start packing them.*0913

*Shoes in the closet, counting both left and right.*0915

*Let us say we got 10 pairs, there are 20 of those.*0920

*R is the number of left handed shoes.*0926

*Left handed shoes sounds a little strange, I will just say left shoes.*0930

*There are 10 left shoes in your closet, assuming that all your pairs match up.*0936

*Let me go ahead and calculate N – R, that is the number of right shoes but that is 20 -10 is still 10.*0942

*N is the number of shoes that you have chosen randomly, when you throw them in a box.*0960

*The number in the box and that is given to us to be 13.*0968

*Y is the number of left shoes that we are interested in.*0979

*Y is 5, 5 left shoes, because we are curious about the likelihood that there are exactly 5 left shoes in the box.*0983

*Let me go ahead and remind you of the formula for the hyper geometric distribution.*0998

*P of Y, it is not hard to remember if you think about what these things are measuring.*1003

*It is R choose Y because it is the number of left shoes available, the number that you are interested in,*1009

*× the number of right shoes available, that is N – R.*1017

*N - R and n – y, that is the number of right shoes that should be in the box × *1022

*all the possible ways of choosing your shoes, that is N choose n.*1028

*I will just fill in all the numbers here.*1035

*R is 10, Y is 5, N - R is 10, n - Y is 13 – 5, that is 8.*1037

*N was 20 and n was 13, 20 choose 13.*1053

*That is all the number of ways that you could have chosen 13 there.*1075

*Again, I did not bother to simplify this down because it will be a lot of factorials.*1080

*I think I will just leave it that way.*1088

*If you want to simplify that down, you could just calculate a bunch of factorials,*1093

*and then do some arithmetic there and get a decimal answer.*1098

*Let me recap and show you where each one of those values came from.*1104

*Each one of these numbers, these parameters for the problem came from somewhere in the problem.*1108

*N is the total number of shoes available in the closet.*1114

*They were 10 pairs which means they were 20 shoes available.*1118

*R is the number of left shoes.*1122

*We figure this analogously to picking a committee of people from a group of women and men.*1125

*Instead, we are picking a box of shoes from a group of left and right shoes.*1133

*R is the number of left shoes that we just picked.*1137

*We picked R to be the number of left shoes.*1143

*We could have switched the role of left shoes and right shoes, and it really would not matter,*1147

*we would end up getting the same answer here.*1150

*The number of left shoes, since there is 10 pairs, there is exactly 10 left shoes that makes *1155

*the number of right shoes to be 20 -10 which is 10.*1161

*That is easy to figure out as well.*1166

*The number of shoes in the box total is 13, that is where that 13 came from, that is n right there.*1168

*Y is the number of left shoes that we are interested in.*1179

*We want to find the probability of getting 5 left choose, that 5 came from that number right there.*1183

*You could switch the roles of left shoes and right shoes.*1189

*You could have keep track of right shoes instead, and that we are giving you the same answer.*1192

*The probability of that Y, I just wrote down the formula for the hyper geometric distribution.*1197

*I do remember this, even though this is kind of a complicated formula, *1203

*it is not hard to remember when you think about what each one of these things it is counting and what each one represents physically.*1206

*I just dropped in all the parameters, r, y, N, n.*1214

*We got some number that you could simplify to a fraction or to a decimal but it did not seem to me to be that relevant.*1221

*We are going to hang onto this example and we are going to keep using this example in problem 4.*1232

*Remember these numbers and we will look in another aspect of this in the next example.*1242

*Example 4, this refers back to example 3.*1248

*If you have not just watched example 3, go back and watch example 3.*1251

*Or at least, read the setup before you look at example 4 and that will make sense.*1255

*Remember back then, we have a shoe closet which has 10 pairs of shoes.*1261

*We start throwing the shoes into a box at random because we are getting ready to move and we are in a hurry.*1266

*We are not going to bother to keep the left shoe with its corresponding right shoe.*1271

*We just throw our shoes into the box and it turns out that there are 13 shoes in the box.*1276

*I'm curious about how many left shoes there might be in the box?*1281

*This is again a hyper geometric distribution, let me remind you of the parameters that we had on example 3.*1287

*This was coming from example 3.*1294

*N was the total number of shoes, that is 20 total number of shoes in your closet.*1296

*r is the number of left shoes, there is 10 left shoes which means that there is 10 right shoes.*1302

*n is the number of shoes in the box which we said back in example 3, we said the box fills up when you got 13 shoes in there.*1313

*Our n is 13.*1323

*I want to know the expected number of left shoes in the box.*1325

*Remember, we sealed up the box, we cannot go and count.*1329

*Let us try to find the expected number of our random variable here.*1332

*Y is the number of left shoes in the box.*1338

*We want to find the expected number of left shoes, E of Y, the expected number of left shoes.*1351

*We have a formula for the expected value of the hyper geometric distribution.*1358

*Let me remind you what it was.*1363

*It is the same as the mean.*1365

*It is n × r/N, that is in this case, n is 13, r is 10.*1368

*I’m just reading these from up above.*1380

*N is 20, the 10 and the 20 simplify down to 13/2.*1382

*13/2 which is 6.5 left shoes.*1390

*It makes perfect sense and another sense is absurd because you cannot have half a shoe.*1400

*You are not cutting your shoes in half.*1406

*It does not really mean that we open the box, there will be 6.5 left shoes in there.*1408

*You either find some whole number shoes, you might find 4 left shoes, you might find 7 left shoes.*1416

*You will not find 6.5 left shoes.*1422

*What it does mean is that if you pack many boxes and there are 13 shoes in each one, *1425

*on average, over the long run you will expect to find 6 1/2 left shoes per box.*1433

*On average, if you add up all the left shoes and divide by the number of boxes.*1440

*Of course, that does not make sense because if you have 13 left shoes,*1446

*remember that in your shoe closet, half of the shoes are left and half of the shoes were right.*1451

*On average, you expect see half of them being left shoes.*1458

*If you have 13 shoes total then on average you expect to see 6 1/2 left shoes.*1461

*Let me recap that problem.*1468

*We took these parameters from examples 3.*1470

*If these numbers look strange to you, just go back and read the setup in examples 3.*1474

*You will see that we had 20 shoes in the closet, 10 left shoes, 10 right shoes.*1479

*We took 13 of them, we threw them into a box.*1485

*The mean number of shoes there, the mean of the number left shoes using the hyper geometric distribution is n × r/N.*1490

*That formula came from our slide about means and standard deviations, earlier on in this lecture.*1499

*I think it was the third slide of this video.*1506

*You can scroll back and see where that comes from.*1508

*I just drop the numbers in 13, 10, and 20.*1511

*Simplify down to 6.5 left shoes which of course, does not make sense because you will find a whole number of shoes in the box.*1513

*But as an expected value, as an average value, it makes perfect sense because out of 13 shoes,*1522

*you can expect half of them to be left shoes and half of them to be right shoes.*1528

*You would expect in the long run, an average of 6 1/2 left shoes in the box.*1532

*Example 5 here is a little more theoretical, it is asking us to use indicator variables and linearity of expectation*1541

*to prove that the expected value of a hyper geometric random variable is n × r/N.*1549

*This one is a little more theoretical, we are going to prove this value.*1559

*We cannot just pull it from the earlier slide.*1562

*Let me show you how this works out.*1565

*Remember the premise of the hyper geometric distribution.*1568

*We are calculating a random variable that represents the number of women on a committee of,*1572

*n was the number of people on our committee.*1593

*We have several parameters here.*1597

*N is the total number of people in the room that we are going to pick from, total number of people.*1602

*Among those total number of people, R is the number of women and N - R is the number of men in the room.*1614

*N – r is the number of men.*1629

*We are going to pick a committee of n people and we want to find the expected number of women.*1632

*There is a very clever way to do this which is to set up indicator variable.*1639

*Let me show you what I mean by indicator variables.*1642

*Let me define Y1, by definition is an indicator variable.*1645

*Let us consider that we are going to pick these people to be on our committee one by one.*1652

*We look around the room and say I want you, you, you, and you, to be on the committee.*1658

*We are picking these people one by one.*1663

*Y1 is going to be an indicator variable that tells us whether the first person on that committee is a woman or not.*1665

*Y1 is defined to be, one if we get a woman on the first pick.*1675

*We pick our first person to be on the committee, Y1 is an indicator variable.*1693

*It is going to be a one if it is a woman, 0 if it is a man.*1698

*It is a little strange but when we say Y1 is the number of women we get on the first choice.*1707

*We either get one woman or we get a man, that is 0 women.*1712

*We will define Y2 to be one, if we get a woman on the second pick.*1716

*The second person we look at.*1730

*If that is a woman, we say Y2 was going to be 1.*1732

*If it is a man, we say Y2 is going to be 0.*1736

*Let us keep on going and we are picking n people to be on this committee.*1742

*We go to Yn here, we define our indicator variables.*1746

*There is one variable for each person on this committee.*1752

*What that means is Y is the total number of women on the committee.*1756

*What that means is it is the number of women we got on the first pick, which is either 1 or 0.*1768

*The number of woman we got on the second pick up to Yn.*1775

*The total number of women, we can count the number of women just by counting all the 1 we got by those indicator variables.*1779

*That breaks down into a sum of these indicator variables.*1787

*In order to find the expected value of Y, the expected number of women, it is the same as the expected value of Y1 + Y2, up to Yn.*1791

*We can use linearity of expectations.*1806

*This is where we are going to use linearity right here, linearity of expectation, very important here.*1809

*These variables are not independent but linearity of expectation does not require that.*1819

*Even though these variables are not independent, if you get a woman on the first pick, *1825

*you are less likely to get a woman on the second because there is fewer women to pick now.*1829

*Even though they are not independent, you can still use linearity of expectation.*1834

*That is the glorious thing about linearity of expectation.*1838

*It breaks up in the expected value of each of these indicator variables.*1843

*What is the expected value of each of these indicator variables?*1848

*Let us think about that, I will give you good way to think about that.*1853

*If you think about just listing Y1, we pick one woman out of a crowd.*1856

*The original definition of expected value is, you look at all the possible values of that variable*1865

*and you multiply that variable × the probability of getting that value.*1873

*This is going back to the original definition of expected value.*1879

*I covered this in one of the very early lectures on probability.*1885

*You can go back and look at some of those early lectures on probability and you will see this.*1890

*What are the possible values of these indicator variables?*1893

*There is only 0 and 1 because we setup here that the indicator variable is always going to be 0 or 1.*1897

*This expands out in to 0 × the probability of 0 + 1 × the probability of 1.*1907

*What is the probability that indicator variable is going to come up 0?*1919

*It is the probability that we get a man because the indicator variable was 0 if we get a man + 1 × *1923

*the probability that we get a woman when we make our first pick.*1934

*I do not care about the 0, the probability of getting a woman.*1940

*How many people were there in the room?*1947

*There were N people in the room and r of those of people is women.*1950

*This is exactly r/N, that is the expected value of one of those indicator variables.*1959

*It is just r/N.*1967

*We can say that all of those indicator variables, they all have the same expected value.*1972

*Each one of these is r/N and there are n of these variables.*1980

*What we get here is n × r/N.*1993

*That is the expected value of our random variable.*2000

*That is the expected number of women on our committee.*2006

*That checks with the value of the mean that I gave you way back on the third slide of this video.*2011

*That is really where that number comes from, now you have the derivation to back it up.*2019

*Now, you hopefully understand it yourself.*2023

*In case that did not make sense, a quick recap here.*2026

*N was the total number of people in the room N.*2029

*r is the number of women which leaves N - R to be the number of men left over.*2033

*We are going to pick a committee of n people and Y is the number of women we get on our committee.*2039

*One way to break that down is to look at our picks one by one.*2048

*We pick this person and then that person and then that person and then that person, to be on our committee.*2051

*Each person we set up this n indicator variable, that is going to be 1 if we get a woman and 0 if we get a man.*2057

*Each person has their own indicator variable and that means the total number of women *2065

*is just the sum of all these indicator variables.*2071

*It is the sum of all the women that we got when we made each one of these picks.*2075

*The expected value is expected value of the sum here is where we use linearity of expectation.*2081

*That is kind of a big deal in probability, let me highlight that to break that up into the expected value of each of these indicator variables.*2089

*We can calculate the expected value of these indicator variables, we just say the only possible values they can take are 0 and 1.*2098

*Using our original definition for expected value, we have 0 × the probability of 0, 1 × the probability of 1.*2107

*We really only need to calculate the probability of 1, which means the probability that we get a woman, *2115

*when we pick a certain person from this room.*2121

*There are R women in the room and n total people in the room, that probability is r/N.*2124

*We fill that in for each of our expected values here, it is the same for every indicator variable.*2137

*We are adding up a bunch of r/N.*2148

*We are adding up n of them and we get n × r/N as our answer.*2152

*That checks with the mean of the hyper geometric random variable that I gave you back earlier on in this lecture.*2162

*That is our last example problem and that wraps up our lecture here on the hyper geometric distribution.*2171

*You are watching the probability videos here on www.educator.com.*2179

*My name is Will Murray, thank you for joining us, see you next time, bye.*2184

3 answers

Last reply by: Dr. William Murray

Tue Sep 2, 2014 7:57 PM

Post by David Llewellyn on August 28, 2014

I don't follow where you get each indicator variable to be equal to r/N.

Y1 is OK but, surely, the probability of picking the gender of the second person depends on your first choice as there is no replacement. If the first choice is a woman then the probability of getting a woman on the second choice is (r-1)/(N-1) but if the first choice was a man then the probability of getting a woman is r/(N-1).

The probability of picking a woman gets even more complex on the third choice as it depends on whether you have picked 2, 1 or 0 women already being (r-2)/(N-2), (r-1)/(N-2) and r/(N-2) respectively.

This trend continues all the way up to the nth choice where depending on how many women have been picked already the probability of picking a woman is (r-n-1)/(N-n-1), ... (r-2)/(N-n-1), (r-1)/(N-n-1), r/(N-n-1).

I can't see how this simplifies to nr/N.

What am I missing?