You may have seen this chart since the start
of 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 understand
this chart.
First, this is not a chart of all coronavirus
cases.
It’s only showing us confirmed cases.
That means each line doesn’t just reflect
the state of the outbreak in a country, but
also how aggressively that country is testing
people for the virus.
Take a look at Japan and South Korea earlier
in the pandemic.
Japan’s outbreak looked pretty small in
comparison.
But the available data on testing shows us
that South Korea had tested vastly more people
than Japan did, even though their population
is 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 understand
case data
without some sense of the testing levels.
And that’s especially true for lower-income
countries
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 the
space 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 amount
of space on this chart
for a certain number of cases.
So the first 100,000 cases take up this much
space
and then the next 100,000 cases get just this much.
The higher the numbers, the more visibly squished
they become on a log scale.
So why do it this way?
Well, let’s take the 5 countries with the
largest outbreaks right now, and rewind them
back to March 17th.
On a linear scale, it looked like things were
pretty 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 that
numbers get big fast.
So it makes sense for numbers to get big fast
on 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 lockdowns
and social distancing, the virus is spreading
at a slower rate than before, which isn’t
very visible on the linear scale.
But keep in mind that the difference between
this dot and this dot
is more than 32,000 people.
And the log chart tends to downplay just how
many 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 for
population size.
When you adjust for population, really small
countries 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 populations
so their curves drop a bit.
But the size of a country doesn’t really
affect the growth rate of its cases, and it
doesn’t tell us much about how much the
country is struggling.
It just pushes smaller countries up on the
chart and tends to hide the fact that the
outbreak is especially bad in certain regions
of bigger countries,
like the state of New York.
And the last thing to know about our chart
is 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 each
other, 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 hit
others.
The world watched as tens of thousands of
cases 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 100th
case, the United States’ did the same.
In a situation where actions taken early can
have a much bigger impact than actions taken
later, time is a crucial factor. and we have
to remember that some governments had more
time than others.
