Psychologists and statisticians often illustrate people’s poor grasp of probability by asking….How many people do you need in a room to have a 50% chance of any two of them sharing the same birthday?

According to the laws of probability the answer is surprisingly small, but is it correct? Let’s put this world famous problem to the test…..

According to the maths, you only need 23 people in a room to get a 50% chance of any two of them sharing a birthday. Squash 57 of them in the same room and the probability of a match apparently rockets to about 99% (for the theory, see here).

Let’s put this theory to the test….

First – if you are reading this, please join in, otherwise the numbers get all screwy.

Second, here is my birthday….17 September (no need for a big present, a card containing money will do)…..

Third, if the first person to read this also happens to be born on the 17 September then they should add a comment saying ‘Match! (17  September)’. However, if they are born on another day they should simply write their birthday (e.g., 12th March). There is no need to add the year you were born.

Fourth, the next person then looks at the two dates (17  September and 12 March) and, again, if they are born on one of these two dates (e.g., 12 March), writes ‘Match! (12 March)‘. If not, they add their birthday to the list.

And so it goes on…..if ANY of the dates already listed matches your birthday then you write ‘Match!’ and say which date it is. Otherwise you add your birthday to the list.

According to the theory, we have a 50% chance of a match with just 23 people and a 99% chance with 57. But will our experiment support the theory?

OK, step forward and share your birthday (and encourage your friends to do the same – we need lots of people for this to work)…..everyone have just one go and no lying!

1. Wai says:

29th January

2. Chrissy Bee says:

25 February

3. Anon says:

16th April

4. Shawn says:

15 January

5. BookGeekGrrl says:

14 December

6. Chris says:

18th January

7. Martin says:

29 June

8. Marina says:

18 October

9. Kristian Z says:

November 5th

10. Jesse says:

28 July

11. Todd says:

18 December (and for what it is worth, the 21st person has the greatest chance of being the first one to say “Match”. It does indeed take 23, though, to have a greater than 50% chance of at least one match.)

12. Jacqueline says:

Match – 29th June

13. CybrgnX says:

17 january

14. Richard Wiseman says:

wow – got our matches very very quickly – carry on and see how many more we get.

15. Elissa Fazio says:

February 12

16. Wendy says:

24 February

17. Mike says:

27 February

18. Jonathan says:

13 November

19. Yumerin says:

23 May

20. Clive says:

19th June.

21. ballookey says:

2nd October

22. GeekGoddess says:

17 November

Match! (5 November)

24. dave says:

27 october

25. Andrea Coelho says:

4th June

26. Scott Fraser says:

9th July

27. Jeremy Miles says:

26 October

28. Marti says:

January 6

29. phoenixphire24 says:

6 April

30. Andrew says:

16 January

31. keving1701 says:

MATCH 18 October

32. Liz says:

31st January

33. yogurt says:

Match! (29th January)

34. trugoy says:

match. Jan-29.

35. Carl says:

23 December

36. Daniel says:

14th October

37. Peter M says:

September 4

38. imeaj says:

7 December

39. Amanda says:

match October 18 (interestingly enough I was on a retreat weekend a few years ago with about 55 other people and there were four of us all with October 18 birthdays)

40. Mark says:

26 March

I’m the 4th person to post who was born on October 18. What do I win? An ice cream cake on my birthday, I hope.

42. Ian says:

(Match!) March 15th

43. Martin says:

February 6th

44. John B. says:

6th Janunary

45. John B. says:

Opps! 6th January

46. Web Maxtor says:

July 10

47. Paul B says:

23 February

48. Dan U. says:

10 December

49. Jason Wurth says:

13th October

50. Mark says:

Match! 21 April

51. Doug says:

26 February

52. Rebecca says:

30 April

53. niteflytes says:

17 July

54. Stephen Williamson says:

Match! (March 15 – three of us so far!)

55. Gabrielle Lynn says:

Match
14 October

56. dml says:

11 August

57. Lara says:

2nd March

58. SerTyrion says:

Match! July 9

59. Jeff says:

12 August. No matches for mine yet 😦

60. Sage says:

19 February

61. a3maniac says:

May 19th

62. Clanory says:

16 February

63. Stewart says:

21 October

64. Kathy g says:

Sept 1

65. Kevin says:

Match – 19th February

66. Frank says:

20 January

67. Frank says:

After about 85 reponses, I am the first to list a birthday on the 20th of any month. Strange.

68. Eve says:

15th November

69. Solus says:

23 August

70. Kristin says:

Match! (21 October)

71. Brodie says:

21 July

72. Rudi says:

18 th November

73. 15th May – only second person in May I think. Perhaps we’re all to busy out being lucky.

74. Joanna says:

3 October

75. Nicola Wray says:

23 March

76. Karsten says:

18 May

77. Josh says:

1st October

78. einalem says:

23 October

79. Angie says:

1st March

80. Stevo says:

11th March

81. KW says:

Match (3rd time) 14th October

82. jazz says:

30th December

83. Anna says:

Match! 22nd February

84. Silje says:

Match! 1st September

85. Stefano says:

Match! (31 January)

86. ranjit says:

Match – 25 October!

87. Pribi says:

21 February

88. Hilary says:

20 May

89. Catherine says:

Match! 24 February

90. Sarah says:

27 September

91. abanazar says:

March 30th

92. Shiftymruzik says:

September 25th.

93. David W says:

1st August

94. Jools says:

28 December

95. Leon Maughan says:

4th April

96. manigen says:

29 July

97. 20th August

I’ve only ever met one person who shares my birthday (and I’m 21 years old)

98. Daniel Eliasson says:

8 April. I had a classmate through grades 1-9 that shared my birthday.

The regular probabilistic answer depends on the assumption that birthdays are distributed evenly across the year. In reality, they aren’t, which in fact suggests you should need even fewer people to find a match.

99. Fergus Gallagher says:

11 May

100. daniel B says:

may 8th

101. 29th November

No matches on here yet, but there were 3 of us at one point in an office of about 40.

The theory assumes an equal distribution of birth dates, which I doubt would actually be the case (I would guess dates like last Saturday’s would result in higher number of conceptions), which should therefore make the match more likely

102. Match! August 20th.

103. Gunnar Pettersen says:

Match! 18th October

104. Liz says:

12th April

105. Sander says:

Match (4th time) 14th October

106. Fergus Gallagher says:

I know twins who share the same birthday. What are the chances of that‽‽‽‽

107. Lucy Martin says:

24th July

108. October 19th (it was early in the morning so I was nearly on the most popular date so far.. So close..)

109. Mona says:

July 23

110. Sophia Collins says:

Match! (May 18th)

111. Rob H says:

Match (14 June)

112. Victor Meldrew says:

26 September

113. Hyanur says:

19 November

114. Emma says:

Match! 3rd person born on 31st January

115. Johanna says:

8 February

116. Mickey Mouse says:

Match Nov 18

117. Richard Todd says:

15 June

118. Stephen Dunn says:

Match 15 January

119. Frankie says:

16th May

120. SteveEwart says:

27 March

121. Naomi says:

Match! 15 June

122. Chloe says:

24th October

123. Rob says:

Match! 29th July

124. Nikodemus Siivola says:

Match! 9th of August.

125. Z says:

Match! February 1st

126. Georgia Lena says:

24th May

From Junior infants to 6th Class (5 to 12 years old) there was one girl in my class of 5 who shared my birthday 🙂

127. Sam says:

Match! 2nd November

128. Arno says:

14th March

129. Darren says:

Match! Jan. 31

130. Shonkhor says:

18th April

131. Alison says:

Match! 31 October

132. Suzaphone says:

Match (10th July)

133. Kaz says:

29th December

134. 20th July

(Only knowingly ever met 1 other person who shares my birthday. I’m 26)

135. Sarah says:

24th January

136. EmilyT says:

Match! 16th April.

137. Helen says:

15th April

138. Micha says:

Match, 14th december

139. Ellen says:

1 April

140. Berber Anna says:

10 February

141. Caroline says:

Match! 8th May

142. Chris says:

Match, September 27

143. Norma says:

27 August

144. Geoff says:

Match! 12 February

145. Caroline says:

24th June

146. JFrank Anastasio says:

(MATCH) 15 July *waves at Luke Randl*

147. Smylers says:

So now we know the birthdays of a bunch of readers of this site. A few people happened to give their age as well, but we could get more by running a perception experiment which proposes that people’s age will affect how they see it and asking for ages in the comments.

Perhaps something similar claiming to be looking for a correlation with where people were born could get us a list of readers’ home towns.

Then you just need one of those ‘what’s your porn star name’-type quizzes which features mother’s maiden name as a component, and we should have enough information to break into a few bank accounts …

148. Brett says:

May 7th

149. Laura says:

Match! (18th November – 2nd match)

150. Flo says:

Match! (23rd July)

151. Jim says:

19 March

152. Ross Bagby says:

Match — 12 August.
There’s an anecdote of a statistician trying this game out on a corporate board meeting and being dismayed at getting no matches — until the unnoticed tea lady suddenly spoke up and said she shared the birth date of one of the big wigs.

153. Bruce says:

4 October

154. phiend says:

20th September

155. Rob West says:

September 11th.

156. Claire says:

MATCH- 8 May

157. Anonymous says:

19th december

158. docpaul says:

12th January

159. Rune says:

28 September

160. soodeebee says:

19 April

161. Flood says:

10th May

162. Michelle says:

Match (24th July)

163. Lux says:

MATCH (7 Dec). – I would hope so with that many dates up there.

164. V1per41 says:

21 August.

165. Lux says:

“I know twins who share the same birthday. What are the chances of that‽‽‽‽”

I know twins who don’t share the same birthday because one was born in the last minutes of the day and the other minutes later on the next day.

166. Carolyn says:

11 April

167. Vat says:

3 Spetember

168. Abigail says:

6th September

169. Suzanne says:

26 July

170. Plain_Gillian says:

Match (9 August)

171. Scott says:

20 February

172. Iconoclast says:

Match… March 30th.

173. Tony says:

Match! (October 3)

174. Shannon says:

Match! (29th January)

175. Megan says:

June 10th

176. Bellman says:

27 April

177. Erik J says:

MATCH! 27 march. (thank you ctrl+f)

178. Trish says:

MATCH! 6 April

179. Jeff says:

November 20

180. Yumerin says:

Matches are starting to pile up!
Let’s see how many we can get in a row!

181. Fizzle says:

June 2

182. Fluffulus says:

8th November

183. Matt says:

Match! (27 September)

184. mannik5000 says:

MATCH! july 10

185. Jim says:

Match – March 19th and first name,
but surely there should be more matches in
July, August, and Sept. than any other months
as in the northern hemisphere, births cluster
around August in a bell curve pattern.

The fact that both my match and I posted
rather late would tend to confirm this.

186. Johnnie says:

17th February

187. Brenda says:

April 23. (As in William Shakespeare, Shirley Temple Black, & President Buchanan)

188. Sally says:

19 August

189. Michael says:

April 7

190. Dave Reckoning says:

27th June

191. Shane says:

Match! 31 December

192. Ian says:

16th June rare birthdate >.< no match

193. scott says:

15th march- MATCH

194. Hugo says:

april 5th

195. Helenka, Florida says:

10th August

196. Kristin says:

MATCH! (April 1) – I’ve known 4 other people to share my birthday, 5 now! 🙂

197. Have commented before so no birthdate here. Asked around and found in my son’s class of 29 there are 3 pairs of birthdays and in my daughter’s class of 25 there is one pair.

198. Riff says:

9 November

199. Julia says:

Match! June 29

200. englishcoach says:

Match! (12 April)

201. ell-ess-eff says:

MATCH – 25 September

hmm, Christmas-made baby – as I was told numerous times while growing up (think about it!).

202. Damian says:

Match! 28th July.

203. Simon says:

7th March

MATCH- May 22 –hi Jewel 🙂

205. Jimmy F says:

MATCH – 19 August

206. Wibke says:

14th September
(no match so far :-()

207. dillene says:

22 July

1. Amanda says:

22 July- Match

208. Diane K says:

17th May

No match here yet, but in my last job working in an office of just four people one of my colleagues shared my birthday.

209. I already posted, so I’m not going to add my date another time.

Just a question: isn’t this little game a little flawed when we do it on a blog instead of a room? What would the chances be of somebody adding a MATCH (they found something in common with another reader and might be more likely to post) compared to the chances of adding a new date?

And how would multiple matches for the same date influence your chance of adding another MATCH for that date?

210. 18th February

(no match on b-day, but I do match with the date that EVERY person so far made their post)

211. skepticalbunny says:

2 February – I have a cousin with the same birthdate, and at least 2 coworkers.

212. John says:

21st November

213. Amanda says:

I already posted once with my date but I wanted to add that I personally know of 6 people with my birthday of October 18 in my town. Also my brother was dating a girl, when he was at college in another city, with the exact same birthday as me down to the year. She was a couple hours older than me.

I agree with the previous post about how the assumption of birthdays being spread out evenly among the year is not quite true. In my primary school class of 53 there were 9 students with birthdays starting at October 15 and ending at October 26 even though the actual dates were from two separate years.

214. I never met before – in 32 years – who has same birthday date as mine.
Maybe now?
🙂

215. Andrea Coelho says:

Still no match for mine (4th June).

216. UponRevelations says:

Match – 9Nov

217. Oriane says:

March 21st. I must have a pretty rare birthday as I’ve never met anyone else with the same birthday as me.

218. Elizabeta says:

Match! Diane K 🙂

17 May, on a Full Moon 😉

219. Maria says:

May 30

220. James says:

23rd November

221. Jamie says:

1st January

222. Alex says:

28th June

223. Geoff says:

Match. 6 September

224. aura says:

match 1st March

225. AnNa says:

Match! 17th December

226. Clive says:

Match! (twice!) 24th July

227. lflcorreia says:

Match! 15th July. I’m the third.

228. Tanya says:

Match – 17th July

229. Chris says:

Match (16th April) – a double match

230. Nick R. says:

Match! (28th July)

231. Norma says:

My Dad, sister-in-law, neighbour, and 2 co-workers share 27 August. Hard to believe no has matched me on this list yet! ;-(

232. Brian_Godfrey says:

26th Feb – Match

233. lipglosser says:

Match! 11th may

234. Donald says:

12th July

235. Karen says:

2 December

236. James says:

3 January

237. Jen says:

Match! April 10th 🙂

238. Jessicajil says:

7th May Match!

239. Catharine says:

16th July

240. Siaron says:

match! 20th January (Frank, you’re not alone!)

241. Frank says:

242. Richie says:

30th june – hardly anyone in june 😥 – however in my first year at uni, when i lived in halls, i shared my flat with someone who had the same birthday, including the year!

243. Merr says:

29 March

244. Dave says:

23 January.

245. JC says:

Match! (17 September)

246. Jen says:

17 June

247. Joseph says:

May 14th

248. me says:

30 November

249. Nik says:

Match! August 3 🙂 my cousin sames the same day but 1 year apart

250. Sean S says:

Match! October 18 – 5th one by my count

251. Bronwyn says:

20 June (no matches yet)

252. Michael W says:

match (twice) 12 April

253. Hannah says:

13 June

254. sikos10 says:

No match yet. =[ – March 31

255. JeffY says:

January 1

256. Alice Cheng says:

October 21st

257. Quackers says:

4th January.

(some people have not said match, even if their bday has already popped up. For example, JeffY, 1st jan has already been said.)

258. Andrew says:

match (april 7)

259. Jason says:

Match! (30th april)

260. Randy says:

October 31, 1951

1. Randy says:

And we have a match!

261. Ljosalfar says:

Match 15th June

262. Levvy Da Dude says:

7th February – I’m guessing that we are getting vewry close to having all the dates?

263. maggie says:

Match! (11th April)

264. lazyluzy says:

October 1 – match

265. Tom Sparke says:

16th August

266. Katy says:

Match! (April 18)

267. StarDUff says:

March 4th

268. AprilO says:

30 January

269. KevA says:

March 16th

270. 'sup says:

Match! May 24th (Hi Georgia Lena)

271. Cosmin Romania says:

Match! (17th June)

272. Ronnie says:

Match! 14 July

273. rob says:

oct 11 never met anyone with the same birthday as me and I am 31

274. maja says:

match (x 4, I think) – 15th June

275. 23rd May… matching two others (including the first May posted)

276. Alex Turner says:

MATCH (x2) 25th September

277. Izaka says:

Match! (16 May)
Finally after searching half of the page =P

278. Julie says:

Match! (25th February)

279. ariane says:

MATCH – 11th February

280. Fig Newton says:

Match 23 December

281. DethEngine says:

5 August

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