Curious observation

[--Pete]

Hi,

I just scanned the member's birthday part of the front page of these forums and saw that no members have a birthday today. Now, I suspect that not all members filled out that part of their bio, but I wonder how many did. With the number of Lurkers we have now, and depending on how many filled in their birth date, is this a probable or improbable event?

--Pete

[DeeBye]

That happened a few days ago as well. I agree that it's kinda odd considering how many people are registered here.

[Chaerophon]

Dangit'! I didn't even check on Tuesday.

As for your question, I filled in mine! Looks like there's around 1770 registered members (as I'm sure you figured out already), so its a statistical possibility, but I personally have the same doubts as you. :)

[pakman]

My birthday came and went already this year...March 3, you can see that in my profile :)

[--Pete]

Hi,

Yeah. The odds of one person not having a birthday on any particular day are 364.25/365.25 = 0.997262149212867898699520876112252 from my trusty windows calculator. So, the odds of all 1770 not having a birthday are simply that number raised to the 1770th power, which if I did it right is 0.0078 or about 3/4 of one percent. Small, but not negligible.

The joker is how many did not give their birthday. It would make a nice problem in a stats course to give the number of people, the number of days when no one had a birthday and then ask what percentage (most likely) of the population did not give a birth date. But there's not much we can do with just a single observation :)

--Pete

[TriggerHappy]

I for one didn't bother entering my date of birth upon registration. Seing as I was born on February 29, I wasn't expecting to show up on that list very often ;) .

[Griselda]

We need to have a "Spend time with your SO day" on June 25. Then, we just have to wait a couple of decades, and things should even out! ;) If not, we could just arrange for a mass power outage on that day.

-Griselda

Edit- Mathematically speaking, I bet there are fairly significant fluctuations in the birth rate throughout the year.

[--Pete]

Hi,

Yep, that would work -- if the events following the eruption of Mount St. Helens is any indicator :)

Mathematically speaking, I bet there are fairly significant fluctuations in the birth rate throughout the year.

I couldn't find anything on this when I tried a search. Possibly someone with better Google Fu (where did all these "Fu" come from?) could get an answer. All I can think of, in the long run, is the seasonal differences. Otherwise, I'd guess that the fluctuations follow the usual square root of the sample size rule.

Aside: Overheard on the World Poker Tour, "Of course I believe in luck. Otherwise how can you explain it when the other guy wins?" ;)

--Pete

[Lady Vashj]

My birthday isn't here yet.

[Cryptic]

Also fouling the math is the human element - my mom was a maternity nurse for 30 years, and sure as clockwork, they had more babies in July and August, every year; and for all but 3 of those years, the record months were in either July or August.

I'll leave it to the reader to deduce what's going on the winter previous, and to wonder why.
:lol:

[LochnarITB]

Also fouling the math is the human element - my mom was a maternity nurse for 30 years, and sure as clockwork, they had more babies in July and August, every year; and for all but 3 of those years, the record months were in either July or August.

I'll leave it to the reader to deduce what's going on the winter previous, and to wonder why.
:lol:

Well, that puts us back to late fall. Maybe, like the squirrels at that time of year, the women are just gathering nuts. I leave the reader to their own definition of nuts. :lol:

[degrak]

I for one didn't bother entering my date of birth upon registration.  Seing as I was born on February 29, I wasn't expecting to show up on that list very often ;) .

I had a few conversations about this before.

I am always curious about those born on this day. Do you celebrate your birthday the day before, the day after, or something else? Do you switch it up?

[AtomicKitKat]

It's actually pretty good odds. I'd say the chance that you WON'T see a birthday listed on the front page, for any given day of the year(barring Feb 29), is about 1 in 20. That is to say, about once every 20 days, you should see nobody's birthday listed. Based on my own casual observation.

Oh, and for the record, it's something like every 32 people=1 pair share their birthday. *looks it up in his Reader's Digest Strange Stories Amazing Facts 2 volume*

Page 194

Birthday Parties
--------------------

It is a fairly well-known fact that in any group of 23 people, there is at least a 50-50 chance that two of them share a birthday.

In a group of 5, the chance that two have the same birthday is just under 3 in 100; for 15 it climbs to just over 1 in 4; and for 23, it is nearly 1 in 2.

The reason lies in a quirk of statisticcs. As the size of a group increases, the number of possible pairs increases aas well - but at a faster rate. In a group of 5, there are 10 possible combinations of 2 people; in a group of 23, there are 253 possible pairs. *AKK's note. It(the number of possible pairings) increases exponentially as the total number of people increases geometrically*

In his book Lady Luck, the mathematician Warren Weaver relates how this curious fact came up in conversation during a dinner party for a number of army officers during World War II.

Most of Weaver's fellow guests thought it incredible that the figure was just 23; they were certain it would have to be in the hundreds. When someone pointed out that there were 22 people seated around the table, he put the theory to the test.

In turn, each of the guests revealed his birth date, but no two turned out to be the same. Then the waitress spoke up. "Excuse me," she said. "I am the 23rd person in the room, and my birthday is May 17, just like the general's over there."

=========================

Why is the above significant? Well, if you consider the fact that we have 500(guesstimate) registered users who posted their DoB in their bios, the odds of any 2(beyond 100%), or 3, or even 6(probably low 10%s) of them sharing a birthday, would be pretty high. This reduces the potential number of "unique" birthdays.

[TriggerHappy]

I am always curious about those born on this day. Do you celebrate your birthday the day before, the day after, or something else? Do you switch it up?

On the 28th, unless it's a leap year. But really I just pick a convinient day of the week close to the real date and have the celebration then.

I suspect other people born on the 29th of February also do the same thing (although I only met one other in real life).

[NiteFox]

Factoid: Welsh people born on the 29th tend to celibrate their "birthday" on the first of March.

Saint David's Day, natch :)

[--Pete]

. . . the odds that any given day be blank are about 25%. Which is why I proposed the problem in this post.

--Pete

[AtomicKitKat]

Well, let's see. Given that there are 365 days a year, and all 500 registered users who posted their DoBs are NOT born on the leap day, we have 500-365=135 people who are guaranteed to share their birthday with someone else.

2 to the power of N-1, where N=total number of people -1

1 persons=(2 power of {1-1=}0)-1=0 pairs
2 persons=(2 power of {2-1=}1)-1=1 pairs
3 persons=(2 power of {3-1=}2)-1=3 pairs
|
|
|
500 persons=(2 power of {500-1=}499)-1=(1.636695303948070935006594848414e+150)-1 possible pairs

So uhh, the odds of any one day being clear are pretty high, like I said. Easily 5%. Or maybe I've confused myself, and a lot of other posters as well? :P

[--Pete]

Hi,

There are, counting leap day, 365.25 days a year. So, the probability that a person's birthday falls on any given day is 1 in 365.25 (1/365.25). The probability that a person's birthday doesn't fall on a given day is thus 1 - 1/365.25 = 364.25/365.25 as I said in the previous post.

Now, consider two people. If they are not twins, the probability of both not having their birthday on a given day is simply
(the probability of the first not having a birthday on that day) times
(the probability of the second not having a birthday on that day).

I looked for a good explanation of this, but all I found where either too technical (if you can read the notation, you probably already know the results) or too superficial. The best I could find was here.

Add a third person. The probability that all three don't have a birthday on a given day is
(the probability of the third not having a birthday on that day) times
(the probability that the first two don't have a birthday on that day) (which we've already worked out.)

Now, this gives us (364.25/365)^1, (364.25/365)^2, (364.25/365)^3 for one, two, and three persons respectively. We can continue to the case of n people by induction. It's a simple argument. We've shown the case for 1 already. Assume that it is true for (n-1), then for n it is
(the probability of the nth person not having a birthday on that day) times
(the probability that the first (n-1) people don't have a birthday on that day)

So, we get (364.25/365)^(n-1) * (364.25/365)^1 = (364.25/365)^n as required.

The odds are easy to calculate given the number of people. It's the number of people that would be, I think, difficult to calculate.

--Pete

[WarLocke]

(where did all these "Fu" come from?)

I would guess from old asian martial arts flicks. You know, "Your Kung Fu is strong," and such.

Also, having a birthday in january sucks because I grew up getting birthday or Christmas presents, but not both. :angry:

Discuss.

[Cryptic]

>>Discuss.

Yup. Sucks.
:P

An ex-girlfriend of mine had her birthday on December 24th (twins, no less). The family used to take *requests* for their gifts at Christmas; anything they didn't get, they'd often ask for again and get in January when the sales were on. How cool is that?
B)