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via io9
Do you ever feel like you’re always running into people with your birthday? It might not be you; it might be the date that you were born. Matt Stiles’ birthday heat map gives us a picture of which birthdays are the most and least common in the United States.

Stiles used a table that ranks birthdays for people born between 1973 and 1999, which you can view at the New York Times. The darker colors indicate the date has a lower-numbered birthday rank and the lighter colors a higher-numbered birthday rank. February 29th and December 25th were ranked 366th and 365th in rank, respectively, while September 16th was the most common birthday. One thing is clear from Stiles’ visualization of the data: card-shopping season has just begun.

Based on this chart, Lane Harrison has created an interactive chart with both the birthdate ranks and the approximate conception ranks

Which brings us to the Birthday Paradox. How many people do you need to have crammed into a room before the odds favor two of you sharing the same birthday? You’d think, given 365 days in a year, that it would need to be around 180. you’d be wrong. The answer is a mere 23 to have a fifty-fifty shot. To bring the probability to ninety-nine percent, you need a crowd of only fifty-seven people.

Explanation here. Warning: math and statistics involved.

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