Using the maker
Type or paste the numbers, separated by commas, spaces or new lines. The sample is 15 test scores. The plot appears as you type, with the key underneath and every step written out below. It copies as plain text, so it stays lined up in any box that uses a fixed-width font.
To compare two groups, paste the first set, leave one blank line, then paste the second. That's it. You get a back-to-back plot, and the first set's leaves read leftwards from the shared stems.
Leaf unit picks which digit is the leaf. Automatic uses the last digit typed, so 3.4 is stem 3, leaf 4. Choose a bigger unit to round long numbers first, which is how you'd plot house prices, say, or the populations of towns, where the last few digits don't matter. Lines per stem splits each stem in two, leaves 0 to 4 on one line and 5 to 9 on the next. That stretches out a plot that's too squashed to show its shape.
How to read a stem-and-leaf plot
Each row is a stem, and each digit after the bar is one value. In the sample, the row 6 | 1 3 3 8 holds 61, 63, 63 and 68. The key at the bottom tells you the place value. Without it, 6 | 1 could be 61, 6.1 or 610.
Turn the page on its side and the rows become bars, so the plot works like a bar chart whose bars are made of the data itself. Long rows are busy. Empty stems are gaps. Nothing is lost. You can read the lowest and highest straight off the ends. Count the leaves in from either end to find the median.
Negative numbers sit on stems below zero. Values between −10 and 0 go on a stem of −0, so −3 and 3 don't share a row.
Where it comes from
Wikipedia traces the stemplot to Arthur Bowley's work in the early 1900s. John Tukey made it popular in his 1977 book Exploratory Data Analysis. It took off in the 1980s partly because it suits a fixed-width font, and that made it easy for computers of the day to print.
Questions
How do you make a stem-and-leaf plot?
Order the numbers. Split each into a stem, the digits before the last, and a leaf, the last digit. Write the stems down the side and each leaf beside its stem, smallest first. Add a key.
How do I do decimals?
Leave Leaf unit on Automatic. With numbers like 4.7 the tenths digit becomes the leaf and the key says 4 | 7 = 4.7.
What is a back-to-back stem-and-leaf plot?
Two sets share one column of stems, one set's leaves going left and the other's going right. It's a quick way to compare, say, two classes' scores. Leave a blank line between the sets to make one.
Why is there a stem with no leaves?
No value fell in that range. The empty stem stays in on purpose, because the gap is part of the picture.
Sources
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