Not just if they cross the median, the median might cross them - for example what if the only people to come back to donate a second time were above the median in their first donation, then even if they all donated the same amount the second time, the median of second-donations would still go up.
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Not just if they cross the median, the median might cross them - for example what if the only people to come back to donate a second time were above the median in their first donation, then even if they all donated the same amount the second time, the median of second-donations would still go up.