Displays of Evidence for Making Decisions – Pt. I

When I finished reading a book chapter by Edward R. Tufte (“Visual and Statistical Thinking: Displays of Evidence for Making Decisions”, chapter 2 from “Visual Explanations: Images and Quantities, Evidence and Narrative”, 1997), I wrote this small piece to share the interesting details behind these visualizations.

How to communicate information the best way, having on account limitations we naturally have (perception, attention, memory) is no easy task, especially if you want to do it in a clean, transparent and comprehensible manner.

In this case, Tufte refers to two situations to illustrate a good and a bad way to communicate information: the cholera epidemic in a 1854 London and the Challenger shuttle disaster in 1986.

In the first situation, Jon Snow made a wonderful detective work trying to discover the source of the epidemic.

According to Tufte, he placed his data in the adequate context to draw cause and effect relations. The original data listed the victim’s names according to their time of death. This could originate displays based n time, or epidemic cronologies like the following graphics. However, time passing is not an explanatory variable, it is even useless when the goal is to discover a strategy of intervention.

Nova imagem

What Jon Snow made was to mark each death on a map, where he also signaled the location of the 13 water pumps of the neighbourhood (in the following image, each trace means the number of deaths in each home, and the water pums are the circles. See next to the D in BroaD Street.)

Deaths from the London Cholera epidemic (John Snow, 1854)

The association between cholera and the water pump localization is evident in this map, and it allows us to compare this scenario with other places with water pumps and no cholera-related deaths.

Besides analysing why people were dying, Snow wondered why other people from the same neighborhood were not dying. He mentioned the case of the brewery, painted in yellow on the map.

You guessed it. The brewery owner allowed his employees to drink some glasses of beer…this way none drank water when thirsty!

Faced with all these evidences, Snow suggests the removal of the Broad Street water pump.

This is where the funny graphical manipulations begin.

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In this daily quantification of deaths, we can see that when the water pump handle was removed, there was already a tendency to the reduction of deaths occuring each day. This happened because people were running away from the epidemic, thus the neighbourhood had less and less inhabitants. So…it’s not clear if the reduction of deaths was exclusively due to the handle removal.

What if the information was plotted this way?

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In a weekly aggregation, it really looks like the reduction of daily deaths is due to the water pump handle removal.

The “time” variable is really sensitive to the choice of intervals. And we can see here how easy it is to manipulate the display in order to communicate certain points of view…

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