### Know urself

**Reader (R)**: I don’t know anything about probability, can I still make it through?

**Professional (P)**: Let’s give it a try. We may have to backtrack a lot at each step.

**R**: What does the phrase “30 random students” mean?

### "When I use a word, it means just what I choose it to mean"

**P**: Good question. It doesn’t mean that we have 30 spacy or scatter-brained people. It means we should assume that the birthdays of these 30 people are independent of one another and that every birthday is equally likely for each person. The author writes this more technically a little further on: “Assume that the birthdays of

*n* people are uniformly distributed among 365 days of the year.”

**R**: Isn't that obvious? Why bother saying that?

**P**: Yes the assumption is kind of obvious. The author is just setting the groundwork. The sentence guarantees that everything is normal and the solution does not involve some imaginitive fanciful science-fiction.

**R:** What do you mean?

**P:** For example, the author is

*not *looking for a solution like this: everyone lives in

Independence Land and is born on the 4

^{th} of July, so the chance of two or more people with the same birthday is 100%. That is not the kind of solution mathematicians enjoy. Incidentally, the assumption also implies that we do not count leap years. In particular,

*nobody* in this problem is born on February 29. Continue reading.

**R**: I don’t understand that long formula, what’s

*n*?

**P**: The author is solving the problem for any number of people, not just for 30. The author, from now on, is going to call the number of people

*n*.

**R**: I still don't get it. So what's the answer?

### Don't Be a Passive Reader - Try Some Examples

**P**: Well, if you want the answer for 30, just set

*n* = 30.

**R**: Ok, but that looks complicated to compute. Where’s my calculator? Let’s see: 365 × 364 × 363 × ... × 336. That’s tedious, and the final exact value won’t even fit on my calculator. It reads:

2.1710301835085570660575334772481e+76

If I can’t even calculate the answer once I know the formula, how can I possibly understand where the formula comes from?

**P**: You are right that this answer is inexact, but if you actually go on and do the division, your answer won’t be too far off.

**R: **The whole thing makes me uncomfortable. I would prefer to be able to calculate it more exactly. Is there another way to do the calculation?

**P: **How many terms in your product? How many terms in the product on the bottom?

**R**: You mean 365 is the first term and 364 is the second? Then there are 30 terms. There are also 30 terms on the bottom, (30 copies of 365).

**P**: Can you calculate the answer now?

**R**: Oh, I see. I can pair up each top term with each bottom term, and do 365/365 as the first term, then multiply by 364/365, and so on for 30 terms. This way the product never gets too big for my calculator. (After a few minutes)... Okay, I got 0.29368, rounded to 5 places.

**P**: What does this number mean?

### Don't Miss the Big Picture

**R**: I forgot what I was doing. Let’s see. I was calculating the answer for

*n *= 30. The 0.29368 is everything except for subtracting from 1. If I keep going I get 0.70632. Now what does that mean?

**P**: Knowing more about probability would help, but this simply means that the chance that two or more out of the 30 people have the same birthday is 70,632 out of 100,000 or about 71%.

**R**: That’s interesting. I wouldn’t have guessed that. You mean that in my class with 30 students, there’s a pretty good chance that at least two students have the same birthday?

**P**: Yes that’s right. You might want to take bets before you ask everyone their birthday. Many people don’t think that a duplicate will occur. That’s why some authors call this the birthday

*paradox.*
**R**: So that’s why I should read mathematics, to make a few extra bucks?

**P**: I see how that might give you some incentive, but I hope the mathematics also inspires you without the monetary prospects.

**R**: I wonder what the answer is for other values of

*n*. I will try some more calculations.

**P**: That’s a good idea. We can even make a picture out of all your calculations. We could plot a graph of the number of people versus the chance that a duplicate birthday occurs, but maybe this can be left for another time.

**R**: Oh look, the author did some calculations for me. He says that for

*n* = 30 the answer is about 71%; that’s what I calculated too. And, for

*n* = 23 it’s about 50%. Does that make sense? I guess it does. The more people there are, the greater the chance of a common birthday. Hey, I am anticipating the author. Pretty good. Okay, let’s go on.

**P**: Good, now you’re telling

*me* when to continue.

### Don’t Read Too Fast

**R**: It seems that we are up to the proof. This must explain why that formula works. What’s this Q(

*n*)? I guess that P stands for probability but what does Q stand for?

**P**: The author is defining something new. He is using Q just because it’s the next letter after P, but Q(

*n*) is also a probability, and closely related to P(

*n*). It’s time to take a minute to think. What is Q(

*n*) and why is it equal to 1

* – *P(

*n*)?

**R**: Q(

*n*) is the probability that no two people have the same birthday. Why does the author care about that? Don’t we want the probability that at least two have the same birthday?

**P**: Good point. The author doesn’t tell you this explicitly, but between the lines, you can infer that he has no clue how to calculate P(

*n*) directly. Instead, he introduces Q(

*n*) which supposedly equals 1

* – *P(

*n*). Presumably, the author will proceed next to tell us how to compute Q(

*n*). By the way, when you finish this article, you may want to deal with the problem of calculating P(

*n*) directly. That’s a perfect follow up to the ideas presented here.

**R**: First things first.

**P**: Ok. So once we know Q(

*n*), then what?

**R**: Then we can get P(

*n*). Because if Q(

*n*)

* = *1

* – *P(

*n*), then P(

*n*)

* = *1

* – *Q(

*n*). Fine, but why is Q(

*n*)

* = *1

* – *P(

*n*)? Does the author assume this is obvious?

**P**: Yes, he does, but what’s worse, he doesn’t even tell us that it is obvious. Here’s a rule of thumb: when an author says

*clearly this is true* or

*this is obvious*, then take 15 minutes to convince yourself it is true. If an author doesn’t even bother to say this, but just implies it, take a little longer.

**R**: How will I know when I should stop and think?

**P**: Just be honest with yourself. When in doubt, stop and think. When too tired, go watch television.

**R**: So why is Q(

*n*)

*= *1

* – *P(

*n*)?

**P**: Let’s imagine a special case. If the chance of getting two or more of the same birthdays is 1/3, then what's the chance of not getting two or more?

**R**: It’s 2/3, because the chance of something not happening is the opposite of the chance of it happening.

### Make the Idea Your Own

**P**: Well, you should be careful when you say things like

*opposite*, but you are right. In fact, you have discovered one of the first rules taught in a course on probability. Namely, that the probability that something will not occur is 1 minus the probability that it will occur. Now go on to the next paragraph.

**R**: It seems to be explaining why Q(

*n*) is equal to long complex-looking formula shown. I will never understand this.

**P**: The formula for Q(

*n*) is tough to understand and the author is counting on your diligence, persistence, and/or background here to get you through.

**R**: He seems to be counting all possibilities of something and dividing by the total possibilities, whatever that means. I have no idea why.

**P**: Maybe I can fill you in here on some background before you try to check out any more details. The probability of the occurrence of a particular type of outcome is defined in mathematics to be: the total number of possible ways that type of outcome can occur divided by the total number of possible outcomes. For example, the probability that you throw a four when throwing a die is 1/6. Because there is one possible 4, and there are six possible outcomes. What's the probability you throw a four or a three?

**R**: Well I guess 2/6 (or 1/3) because the total number of outcomes is still six but I have two possible outcomes that work.

**P**: Good. Here’s a harder example. What about the chance of throwing a sum of four when you roll two dice? There are three ways to get a four (1-3, 2-2, 3-1) while the total number of possible outcomes is 36. That is 3/36 or 1/12. Look at the following 6 by 6 table and convince yourself.

1-1, 1-2, 1-3, 1-4, 1-5, 1-6

2-1, 2-2, 2-3, 2-4, 2-5, 2-6

3-1, 3-2, 3-3, 3-4, 3-5, 3-6

4-1, 4-2, 4-3, 4-4, 4-5, 4-6

5-1, 5-2, 5-3, 5-4, 5-5, 5-6

6-1, 6-2, 6-3, 6-4, 6-5, 6-6

What about the probability of throwing a 7?

**R**: Wait. What does 1-1 mean? Doesn’t that equal 0?

**P**: Sorry, my bad. I was using the minus sign as a dash, just to mean a pair of numbers, so 1-1 means a roll of one on each die - snake eyes.

**R**: Couldn’t you have come up with a better notation?

**P**: Well maybe I could/should have, but commas would look worse, a slash would look like division, and anything else might be just as confusing. We aren’t going to publish this transcript anyway.

**R**:

** **That’s a relief.

** **Well, I know what you mean now. To answer your question, I can get a seven in six ways via 1-6, 2-5, 3-4, 4-3, 5-2, or 6-1. The total number of outcomes is still 36, so I get 6/36 or 1/6. That’s weird, why isn’t the chance of rolling a 4 the same as for rolling a 7?

**P**: Because not every sum is equally likely. The situation would be very different if we were simply spinning a wheel with the sums 2 through 12 listed in equally spaced intervals. In that case, each one of the 11 sums would have probability 1/11.

**R**: Okay, now I am an expert. Is probability just about counting?

**P**: Sometimes, yes. But counting things is not always so easy.

**R**: I see, let’s go on. By the way, did the author really expect me to know all this? My friend took Probability and Statistics and I am not sure he knows all this stuff.

**P**: There’s a lot of information implied in a small bit of mathematics. Yes, the author expected you to know all this, or to discover it yourself just as we have done. If I hadn’t been here, you would have had to ask yourself these questions and answer them by thinking, looking in a reference book, or consulting a friend.

**R**: So the chance that there are no two people with the same birthday is the number of possible sets of

*n* birthdays without a duplicate divided by the total number of possible sets of

*n* birthdays.

**P: **Excellent summary.

**R: **I don’t like using

*n*, so let me use 30. Perhaps the generalization to

*n* will be easy to see.

**P**: Great idea. It is often helpful to look at a special case before understanding the general case.

**R**: So how many sets of 30 birthdays are there total? I can’t do it. I guess I need to restrict my view even more. Let’s pretend there are only two people.

**P**: Fine. Now you’re thinking like a mathematician. Let’s try

*n* = 2. How many sets of two birthdays are there total?

**R**: I number the birthdays from 1 to 365 and forget about leap years. Then these are the all the possibilities:

1-1, 1-2, 1-3, ... , 1-365,

2-1, 2-2, 2-3, ... , 2-365,

...

365-1, 365-2, 365-3, ... , 365-365.

**P**: When you write 1-1, do you mean 1-1 = 0, as in subtraction?

**R**:

** **Stop teasing me. You know exactly what I mean.

**P**:

** **Yes I do, and nice choice of notation I might add. Now how many pairs of birthdays are there?

**R**: There are 365 × 365 total possibilities for two people.

**P**: And how many are there when there are no duplicate birthdays?

**R**: I can’t use 1-1, or 2-2, or 3-3 or ... 365-365, so I get

1-2, 1-3, ... , 1-365,

2-1, 2-3, ... , 2-365,

...

365-1, 365-2, ... , 365-364

The total number here is 365 × 364 since each row now has 364 pairs instead of 365.

**P**: Good. You are going a little quickly here, but you’re 100% right. Can you generalize now to 30? What is the total number of possible sets of 30 birthdays? Take a guess. You’re getting good at this.

**R**: Well if I had to guess, (it’s not really a guess, after all, I already know the formula), I would say that for 30 people you get 365 × 365 ×... × 365, 30 times, for the total number of possible sets of birthdays.

**P**: Exactly. Mathematicians write 365

^{30}. And what is the number of possible sets of 30 birthdays without any duplicates?

**R**: I know the answer

*should* be 365 × 364 × 363 × 362 × ... × 336, (that is, start at 365 and multiply by one less for 30 times), but I am not sure I really see why this is true. Perhaps I should do the case with three people first, and work my way up to 30?

**P**: Splendid idea. Let’s quit for today. The whole picture is there for you. When you are rested and you have more time, you can come back and fill in that last bit of understanding.

**R**: Thanks a lot; it’s been an experience. Later.