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Common Birth Dates (Posted on 2021-09-29) Difficulty: 2 of 5
It has been noted that Roger Ebert and Paul McCartney were born on the same day (year, month and day). How many such coincidences should you expect?

To make things specific, assume there are 1000 celebrities of such stature and recency, and that whatever their average age is, the standard deviation of their ages is 12 years and follows a normal distribution. Of course the date is rounded to the day; don't worry about the hours.

Feel free to vary the assumptions for bonus answers.

  Submitted by Charlie    
Rating: 4.3333 (3 votes)
Solution: (Hide)
The average result (1000 trials in each of several runs) of the following simulation is 31, to the nearest integer.

for trial=1:1000
for i=1:1000
   d=round(randn*12*365.25);
   n(i)=d;
end
n=sort(n); ct=0;
for i=2:1000
   if n(i)==n(i-1)
      ct=ct+1; 
   end
end
tot=tot+ct;
end
tot/trial

The randn function/pseudovariable provides a random number drawn from a normal distribution with a mean of zero and standard deviation of 1.

Here's a list of 74 pairs from

https://www.ranker.com/list/people-born-the-same-day/celebrity-lists

I've sorted them by year, but not within year.

Abraham Lincoln & Charles Darwin - February 12, 1809
Barbara Stanwyck & Orville Redenbacher - July 16, 1907
Ralph Ellison & Harry Caray - March 1, 1914
Judy Holliday & Jane Russell - June 21, 1921
Bettie Page & Aaron Spelling - April 22, 1923
Pol Pot & Malcolm X - May 19, 1925
Lenny Bruce & Margaret Thatcher - October 13, 1925
Marilyn Monroe & Andy Griffith - June 1, 1926
David Attenborough & Don Rickles - May 8, 1926
Maya Angelou & Estelle Harris - April 4, 1928
Eddie Fisher & Jimmy Dean - August 10, 1928
Fats Domino & Ariel Sharon - February 26, 1928
Stephen Sondheim & Pat Robertson - March 22, 1930
Mikhail Gorbachev & Tom Wolfe - March 2, 1931
Michael Caine & Quincy Jones - March 14, 1933
Billy Dee Williams & Merle Haggard - April 6, 1937
Natalie Wood & Diana Rigg - July 20, 1938
Lee Harvey Oswald & Mike Ditka - October 18, 1939
James Caan & Nancy Pelosi - March 26, 1940
Pete Best & Duck Dunn - November 24, 1941
Julie Christie & Pete Rose - April 14, 1941
Paul McCartney & Roger Ebert - June 18, 1942
Garrison Keillor & Tobin Bell - August 7, 1942
Newt Gingrich & Barry Manilow - June 17, 1943
Rudy Giuliani & Gladys Knight - May 28, 1944
Stockard Channing & Jerry Springer - February 13, 1944
Chris Matthews & Ernie Hudson - December 17, 1945
Patch Adams & John Fogerty - May 28, 1945
Van Morrison & Itzhak Perlman - August 31, 1945
Anthony Daniels & Alan Rickman - February 21, 1946
Patty Duke & Jane Birkin - December 14, 1946
Freddie Mercury & Loudon Wainwright - September 5, 1946
Christa McAuliffe & Terry Bradshaw - September 2, 1948
Rhea Perlman & Al Gore - March 31, 1948
Lester Bangs & Ted Nugent - December 13, 1948
Rick Springfield & Shelley Long - August 23, 1949
Linda Lovelace & George Foreman - January 10, 1949
John Hughes & Cybill Shepherd - February 18, 1950
Kirstie Alley & Rush Limbaugh - January 12, 1951
Geddy Lee & Tim Gunn - July 29, 1953
Stevie Ray Vaughan & Al Sharpton - October 3, 1954
Madonna & Angela Bassett - August 16, 1958
Weird Al Yankovic & Nancy Grace - October 23, 1959
Sarah, Duchess of York & Emeril Lagasse - October 15, 1959
Daryl Hannah & Julianne Moore - December 3, 1960
John F. Kennedy Jr. & Amy Grant - November 25, 1960
Ralph Macchio & Jeff Probst - November 4, 1961
Manute Bol & Flea - October 16, 1962
Michael Jordan & Larry the Cable Guy - February 17, 1963
Rob Schneider & Dermot Mulroney - October 31, 1963
Michelle Obama & Michelle Fairley - January 17, 1964
Steve Wilkos & Juliette Binoche - March 9, 1964
Trey Anastasio & Monica Bellucci - September 30, 1964
Wanda Sykes & Bret Easton Ellis - March 7, 1964
Kurt Cobain & Lili Taylor - February 20, 1967
Jamie Foxx & NeNe Leakes - December 13, 1967
Boris Becker & Mark Ruffalo - November 22, 1967
Jason Bateman & Dave Grohl - January 14, 1969
Alfonso Ribeiro & Luke Wilson - September 21, 1971
Martin Freeman & David Arquette - September 8, 1971
Henry Thomas & Eric Stonestreet - September 9, 1971
Monica Lewinsky & Nomar Garciaparra - July 23, 1973
Neve Campbell & Lena Headey - October 3, 1973
Angelina Jolie & Russell Brand - June 4, 1975
Eva Longoria & will.i.am - March 15, 1975
John Cena & John Oliver - April 23, 1977
Shannon Miller & Robin Thicke - March 10, 1977
Heath Ledger & Natasha Lyonne - April 4, 1979
Christina Ricci & Gucci Mane - February 12, 1980
Joseph Gordon-Levitt & Paris Hilton - February 17, 1981
Danielle Fishel & Craig David - May 5, 1981
Aziz Ansari & Emily Blunt - February 23, 1983
Jonah Hill & Lucy Pinder - December 20, 1983
Garrett Hedlund & Paz de la Huerta - September 3, 1984

You may judge for yourself how well the years and degree of celebrity match the criteria. In 35 of the pairs I recognized both names. I'm sure younger people would recognize some of the later-born names, but fewer of the older.

In relation to the familiar birthday problem of how many people you need to be confident of at least one common birthday, this extends such to year consideration. The 1000 could be attendees at a concert in an approximately one generation cohort, and you could expect about 31 pairs of people born on the exact same day. At a concert with 555 people in the same generational cohort, you're virtually guaranteed to have two in the audience born on the exact same day, with the expected number of such matches being about 9.8.

Comments: ( You must be logged in to post comments.)
  Subject Author Date
Puzzle AnswerK Sengupta2022-05-11 22:00:26
No SubjectChris Peters2022-02-25 03:22:35
No SubjectJoe2021-10-28 07:33:38
An interesting extensionMath Man2021-10-02 19:01:59
On the age distributionJer2021-10-02 17:14:57
Another rough estimateSteve Herman2021-10-02 09:54:35
Some ThoughtsRough estimateJer2021-10-01 09:00:11
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