Statistics / Unit 1: Data and Displays
1.4 Dot Plots and Stem Plots
Notes
A dot plot puts one dot above a number line for each value, stacking dots when values repeat. A stem plot splits each value into a stem (leading digits) and a leaf (last digit).
- Both keep every individual value visible and show shape, center, spread and gaps.
- A stem plot needs a key, such as $4 \mid 7$ means $47$. List leaves in increasing order.
- Example: the values 12, 15, 21, 23, 23, 38 give stems $1 \mid 2\ 5$, then $2 \mid 1\ 3\ 3$, then $3 \mid 8$.
- Best for small to moderate data sets. A histogram is better for large ones.
Practice
A stem plot has these rows: $3 \mid 2\ 5\ 9$, $4 \mid 0\ 0\ 7$, $5 \mid 1$. The key is $4 \mid 7 = 47$. How many values are at least 40?
The values at least 40 are 40, 40, 47 and 51.
A dot plot of pets per student shows 3 dots at 0, 5 dots at 1, 4 dots at 2, 2 dots at 3 and 1 dot at 4. How many students have at least 3 pets?
$2 + 1 = 3$ students (the dots at 3 and at 4).
An advantage of a stem plot over a histogram is that it:
A histogram groups values into bins, but a stem plot keeps the actual data.
Ready for more? Take the Unit 1 test.