A (happy) birthday problem (Run Report #98)

As we approach College Park's parkrun's 2nd birthday (October 20th), it is perhaps appropriate that the past few parkruns have coincided with birthdays for parkrunners. Parkrun #97 was run director Brian Murphy's birthday (and Roosevelt Island parkrun's 2nd), and this parkrun (#98) coincided with the birthday of two parkrunners': Meredith Phillips and Lois Zukowski (event director Andrea's mother, visiting from Michigan).

Meredith and Lois (p = 99.9999999%)Now, you might marvel at the fact that two parkrunners (out of 139 parkrunners) have the same birthday -- "how rare is that?!" (Well, this is a known mathematical problem: it's so well known, it's even got a name -- the "Birthday Problem" -- and its own Wikipedia page. Scroll to the bottom of this post to see a summary.)

Also remarkable is that on Saturday Lois became twice the age of Meredith. You might now wonder, what is the probability of two parkrunners out of 139 sharing the same birthday AND one being twice the age of the other? Well, that's a much harder problem; there is no Wikipedia page for it. We will waive the registration fee for all parkruns for anyone who sees a way to solve this problem.

We also marked other milestones, in addition to birthdays: At 90 parkruns each, Kirk Gordon and John Jensenius (with a PB) are now neck-to-neck in hitting the 100-parkrun milestone (Lisa Wilson is right behind, at 87). Thelma Cordón and frequent visitor Adrian Dover have completed 10 parkruns at College Park, while Laura Frese, Lloyd Rawley, Leah Baker, Doug Kerr, Jen Charles, Maria Ialacci (and a PB!), Elizabeth Bast have done five each.



Turnout: as mentioned, we had 139 parkrunners on Saturday  -- look at that!CP-parkrun-98-9

We hosted a few visitors, including Travis Helms, from Roosevelt Island and Denis Primakov from Kensington parkrun.


Our numbers were further boosted by two groups of visitors. The first was a wedding party -- congratulations to Rhonda Summey and fiancé, and thanks for having us as part of your celebrations! The two of you win the Best Dressed Award.

CongratsThe second was the cross-country team from Elizabeth Seton High School; thanks to coaches Louisa and Mike Puffett for bringing them along. You all lowered the average age of this parkrun turnout. CP-parkrun-98-3


We also saw plenty of PBs, despite the humid weather. A few highlights: Maria Beckert and mom Laura set PBs, as did sisters Colette Foley and Elizabeth. Colette's PB is a new under-11 girls record for this parkrun. Carly Mills set a fourth PB in 4 weeks, breaking the 30-minute barrier, cheered on by her sister Erin Munsell, who was timekeeping.

John and Emily Jensenius collectively have two PBs in two weeks (their secret: switch stroller-pushing duties). Diego Aguilera Kelley and Benno Wien set PBs ontheir 9th parkrun; we will see if they will set another one on their 10th.


Special thanks to our volunteers, without whom parkrun is impossible: Lisa Wilson and Hump Plotts took up their positions on the trail; LisaAndrea Zukowski and Colin Phillips took photos; Meredith Phillips was the tail-walker and kept everyone safe. Thanks to Anna Tinnemore for walking with Julie Russell. At the finish line, Larry Washington, Erin Munsell, and Joel Goldberg handled timing, Ben Brosch handed out tokens, Jim Cantwell and Wei Chen scanned them. At the Board and Brew, two mathematics professors -- Lou Shapiro and Larry -- took on the intellectually demanding task of sorting the finish tokens in ascending order.

Finish line


Sorting tokens

Lastly, a couple of announcements.

First, important dates that are coming up:

  • Sept 15th: our 100th parkrun.
  • October 6th: College Park Day (at the College Park Aviation Museum). We have a booth, and are looking for volunteers to staff it -- the more the better. Please drop Andrea a line if you are interested: collegeparkoffice@parkrun.com
  • October 20th: second birthday of College Park parkrun! (How many parkrunners will be celebrating their birthdays that day?)

Second, reminders:

  • We lost a token on Saturday (our two mathematicians confirmed it). If you accidentally brought it home with you, please bring it back --- we re-use them every week.
  • Please remember to bring your barcode (paper copies, please)! Having your barcode with you simplifies the results processing, so you get your results earlier.

See you Saturday!

Nick Huang
Run Director


[Summarizing the Wikipedia page: it's actually almost certain that two people in a group of 139 have the same birthday. With a group of 23 randomly-selected people, there is already a 50% probability that there are two people in this group with the same birthday. With a group of 100, the probability goes up to 99.99997%. (If this probability seem incredibly high to you, note that we are in effect asking, how likely is it that in a group of 100 people, each person has a different birthday? There are only 366 birthdays to choose from, including February 29.)]