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Tufte Presenting information
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Good representation of data Cholera epidemic in London in 1854. John Snow, a physician, had a theory  that people could be getting sick from  water from a particular pump. The outbreak of Cholera was on 8-31. The water from the pump in the area  was tested on 9-3, but nothing was  found.
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Snow on the Cholera outbreak “This absence of evidence, however,  was not evidence of absence”    Snow could not think of another  plausible hypothesis, so he did not  give up on the idea of the water being  the problem. After further testing, he found the  results were not consistent: the amount  of impurities in the water would vary.
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Snow on the Cholera outbreak Snow got a list of everyone who had  died from Cholera in the outbreak:  83  people  He plotted the location of those people  on a map. It turned out that it really did look  like there was a close link between  the location of the Cholera deaths and  the Broad Street pump.
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Snow on the Cholera outbreak The map did show the deaths  clustered around the Broad Street  pump, for the most part. There were, however, 10 deaths that  were clearly closer to a different water  pump than the Broad Street pump. So, Snow went door to door and  inquired into details in each of these  cases.
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Snow on the Cholera outbreak He found out: 5 of the households had routinely gotten  their water from the Broad Street pump  out of preference. 3 of the other deaths were of children  that went to school near the Broad Street  pump 2 of whom it was known had had water from  the pump, the other quite plausibly had  water from that pump
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Snow on the Cholera outbreak That left 2 deaths that could not be  traced back to the Broad Street pump. But this should not be troubling.  That number of deaths in fact was the  normal rate of Cholera at the time,  before the outbreak.
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Snow on the Cholera outbreak Snow did not stop there, he also looked  into the details of the cases that  were   nearest the Broad Street Pump Of those 63 people: 61 were reported to have gotten their  water from that pump at least  sometimes 6 Snow could not get information about  6 it was reported did not drink from  that pump  
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Snow on the Cholera outbreak All of this information confirmed  Snow’s hypothesis—the rate of  Cholera was not high (not higher than  normal) except among people who  drank from the Broad Street Pump
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