Stem and leaf plots

Leon uses a heart rate monitor to find out what his heart rate is when he is out running. The heart rate monitor counts how many times Leon’s heart beats every minute. Here, the results are displayed in a table. It shows his heart rate in beats per minute, after one minute, two minutes, three minutes… all the way to 19 minutes.. Leon wants to sort the results in order to get a clearer picture of when his heart rate was highest and lowest, and where it hovered for the longest time.

First he sorts the table from the lowest heart rate to the highest. But Leon is not really happy. He thinks the table is too long and could still be clearer. To make the table shorter, he first divides the results into hundreds, tens, and ones. Then he makes two columns: hundreds and tens in one column, and ones in the other.

A heart rate of 112 then becomes ‘one-one’ in the hundreds and tens column, and two in the ones column. Like this: Then the next result. ‘One-two’ in the hundreds and tens column, and eight in the ones column. Next, 134, becomes ‘one-three’ in the hundreds and tens column, and four in the ones column. And now 139. But wait!

Here, Leon doesn’t need to write ‘one-three’ in the hundreds and tens column. It’s already there from the last result, 134. Instead, he simply writes the ones value, ‘9’, next to the four. This way, he notes both results, 134 and 139. Leon continues to fill in the results..

There are two results of 161. So next to ‘one-six’ in the hundreds and tens columns, Leon writes two ones. He fills in all the other results. Next, he draws a vertical line between the ‘hundreds-tens’ and the ones. The new table looks like a tree with a ‘stem’, of hundreds and tens.

And from the stem grows branches with ‘leaves’ of ones. Data presented this way is called a stem and leaf plot. The stem and leaf plot is shorter than the table Leon started with. So it is easier to find the lowest and highest heart rate. We can also easily see where the heart rate falls most commonly — it’s the longest branch: the 160s..

And look here! If Leon rotates the stem and leaf plot 90 degrees, it actually looks like a bar graph — no need to use a pen or ruler! And with bar graphs, you can.. I do not have time for math now. I'm working out!

Well, okay, then the movie is over then.. Bar charts, they can be used for a lot..