Main content

## Analyzing one categorical variable

# Reading pie graphs (circle graphs)

## Video transcript

For the past year,
a travel agency has collected data about the
number of individual tickets that it sells for its signature
product, a Mediterranean cruise. The monthly data on ticket
sales is shown below. What are the best and worst
months for cruise sales? So what they have
given us, this diagram. This is usually called
a pie chart or pie graph because it looks
like a pie that's sliced up into a
bunch of pieces. Sometimes this is
called a circle graph. But pie graph is
much more common. And then they say it's
monthly ticket sales. So each of these
slices represent the sales in a given month. So for example, this
blue slice over here represents the sales in January. And the way that a
pie chart is set up, each slice is bigger
or smaller, depending on what fraction of the
whole it represents. So for example, they're
telling us in January they sold 18% of the total
year's ticket sales in January. So if you add up all
of these percentages, it should add up to 100%. And not only do they tell
us that they sold 18%, but the slice of
this pie should be 18% of the area
of the entire pie. It is literally 18% of the pie. If you were to eat
this slice, you would have eaten 18% of the pie. Now with that out of
the way, let's think about their questions. What are the best and the
worst months for cruise sales? So the best month is obviously
the month where they sell, where they have the largest
percentage of their tickets were sold. And actually, I started with
January, and if you just-- and this is what's
neat about pie graphs. You wouldn't even have
to look at the numbers, January just jumps out as
the biggest slice of pie. If you didn't even
see the numbers, if you couldn't even read,
and you just looked at this and someone said what is
the largest slice of pie? You would immediately
say this is clearly the largest slice of
pie right over there. And so that is actually the
best month for cruise sales because they sold 18%. You see this 18%
is larger than all of the other
percentages over here. But it's clear, just by
looking at the graph, this is the largest slice. Now what's the worst month
for cruise ticket sales? Well, then we just have to
find the thinnest slice of pie. And if we look over
here, the slices of pie get pretty thin out down here. This is in the summer, in
June and July, and in May. But the smallest are
actually July and June. And this is where the
numbers become useful. Because when you just
eyeball it, you're not sure, hey, are these exactly the same? Or do they just look
exactly the same? And that's where these
numbers are valuable. And based on the data
they've given us, it looks like these are
tied for the worst in terms of ticket sales. In both of these months they
sell only 3% in each month. So the worst months for cruise
sales are July and June. July and June are
tied for the worst. The best is clearly January,
and then after January the next best-- they're
not really asking us that, but since we have the
pie chart in front of us, might as well ask ourselves
that, what's the next best? Well it looks like November is
slightly better than December. But in general,
it's pretty clear that the winter months do much
better in terms of cruise sales than these over here. And I guess people are looking
to get away from the cold.