You may have seen this chart since the startof the coronavirus pandemic.
In one image, it appears to capture the state of each nation’s battle in the global war against the virus.
But like all data visualizations, its design tends to emphasize some things and hides others.
So here are 4 things we need to know to understandthis chart.
First, this is not a chart of all coronaviruscases.
It’s only showing us confirmed cases.
That means each line doesn’t just reflectthe state of the outbreak in a country, but also how aggressively that country is testingpeople for the virus.
Take a look at Japan and South Korea earlierin the pandemic.
Japan’s outbreak looked pretty small incomparison.
But the available data on testing shows usthat South Korea had tested vastly more people than Japan did, even though their populationis less than half as big.
And now, as Japan slowly increases their testing, the outbreak there looks more worrying.
It’s a good reminder that we can’t understandcase data without some sense of the testing levels.
And that’s especially true for lower-incomecountries where we know their testing capacity is limited.
The second thing to know is that the scale for the y-axis on this chart is a bit different from most charts.
It’s called a logarithmic or log scale.
On a typical linear scale, you divide thespace by adding the same value over and over.
The log scale is made by multiplying a value, in this case, ten.
100 times 10 is 1000, times 10 is 10, 000, times 10 is 100, 000.
That means that there’s no fixed amountof space on this chart for a certain number of cases.
So the first 100, 000 cases take up this muchspace and then the next 100, 000 cases get just this much.
The higher the numbers, the more visibly squishedthey become on a log scale.
So why do it this way? Well, let’s take the 5 countries with thelargest outbreaks right now, and rewind them back to March 17th.
On a linear scale, it looked like things werepretty bad in Italy but the others were doing better.
The log scale offered a much clearer warning: we were all on the same path of exponential growth.
It’s the nature of infectious disease thatnumbers get big fast.
So it makes sense for numbers to get big faston the chart too.
Fast forward a few weeks and the linear scale shows cases climbing and climbing while the log chart shows curves that are flattening.
As governments have implemented lockdownsand social distancing, the virus is spreading at a slower rate than before, which isn’tvery visible on the linear scale.
But keep in mind that the difference betweenthis dot and this dot is more than 32, 000 people.
And the log chart tends to downplay just howmany more confirmed cases there are in the US than in the other countries.
Which brings us to the third thing to know about our chart: it doesn’t account forpopulation size.
When you adjust for population, really smallcountries like Iceland and Luxembourg appear to have the biggest outbreaks for their size, which may reflect higher testing rates.
The US and China have much bigger populationsso their curves drop a bit.
But the size of a country doesn’t reallyaffect the growth rate of its cases, and it doesn’t tell us much about how much thecountry is struggling.
It just pushes smaller countries up on thechart and tends to hide the fact that the outbreak is especially bad in certain regionsof bigger countries, like the state of New York.
And the last thing to know about our chartis that the x-axis doesn’t plot time by the date, but by the number of days since the country recorded 100 confirmed cases.
For Italy, that was February 24.
For Turkey, March 19.
When they’re all layered on top of eachother, it allows us to compare the trajectory of the outbreaks, but it tends to obscure the fact that the pandemic hit some countries before it hitothers.
The world watched as tens of thousands ofcases appeared in China.
Then big outbreaks in South Korea, Italy, and Iran, sent a message about what was to come.
Two weeks after South Korea reported its 100thcase, the United States’ did the same.
In a situation where actions taken early canhave a much bigger impact than actions taken later, time is a crucial factor.
and we haveto remember that some governments had more time than others.