Dating someone with the same birthday astrology
Next one should figure out what the chance is of being born in the same year.Just to give an impression of this chance I'd start with the rule of thumb as presented by xkcd which gives a dating age range of $\pm \ age-\frac 7$, where age $=$ age you started dating. However, the chances of dating someone from the exact same year are far greater due to a flaw in the educational system where pooling is done by the date of conception.If it's an event specified before the fact, you can simply break it down: The chance that your boyfriend was born the same year as you is actually very high (especially given many situations tend to bring people of very similar age together); it's a very difficult probability to calculate, though, without data.So if you had some good estimate of P(Same year), you can calculate the overall probability reasonably well.With enough analyses, the probability of finding a significant pattern gets close to one even when alpha is small.
If you had that probability, P(Same day and same year) = P(Same year) $\times$ P(Same day|same year).
I'd guess that P(same year) is roughly of the order of 0.1 to 0.2, but that's just a guess. I've never understood this in terms of evolution, but it seems to be highly correlated.
[Edit: Jeromy gives a figure based on actual data, which turns out to be about 17%.] My guess would be slightly lower. However the percentage of same-year relationships is likely much higher in high school.
But most of those hardly even qualify for a relationship According to wikipedia, 33.2% of married couples in the United States differ in age by less than one year.
Thus, a baseline estimate for sharing the same date of birth would be the above statistic divided by two (because it captures 2 years) for sharing the same year multiplied by the probability of sharing the same birthday: $$P(DOB_i=DOB_j)\approx \frac \times \frac = 0.00045$$ Or roughly 1 in 2200.
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Your question got me thinking, and I realized my girlfriend and I also have a strange birthday coincidence. 1 Very reminiscent of the Richard Feynman quote: "You know, the most amazing thing happened to me tonight. Therefore, the probability of differing 432 days is likely not identical to the probability of differing 0 days (although with such a small age difference, we can probably approximate those probabilities as equal).