3
A dot plot of counted beans
Discover Mathematics P3: Term 1, Lessons 1–10
GoalMake and read a dot plot from counted data, with a title, a labelled number line and a key.
New words
dot plot · a graph with one dot above a number line for each piece of data (also called a line plot)data · numbers or facts that we collectkey · a note that tells what one dot meansnumber line · a line with equally spaced numbers
Explanation
Mostafa counts the beans in 10 small bags and gets 7, 8, 8, 9, 9, 9, 9, 10, 10 and 11. That list is hard to read, so he makes a dot plot. He draws a number line from the smallest count to the biggest. For every bag he puts one dot above its number. Then he gives the plot a title, Beans in 10 bags, labels the number line, and writes a key: each dot is 1 bag. The tallest stack of dots shows the number that happened most often: 4 bags had 9 beans.
Worked examples
| Beans in a bag | 7 | 8 | 9 | 10 | 11 |
|---|---|---|---|---|---|
| Number of bags | 1 | 2 | 4 | 2 | 1 |
Nine children count the pencils in their pencil cases. The plot shows 1 case with 2 pencils, 2 cases with 3, 4 cases with 4, 1 case with 5 and 1 case with 6. The most common number is 4 pencils. The number of dots is 1 + 2 + 4 + 1 + 1 = 9, one for each child.
A bar graph can show the same data. Here the bar for 4 pencils is the tallest, just like the tallest stack of dots. A dot plot shows every child as a dot, and a bar graph shows each total as one bar. Both tell us that 4 is the most common number.
Class activity
Dot plot with real things. Make 8 small piles of dried beans or buttons, with between 3 and 9 in each pile. Count each pile and write the numbers down. Draw a number line on paper and put one dot for each pile. Give your plot a title and a key. Which number appears most often?
Practice
1
In how many matches did the team score 2 goals?
4 matches.
2
How many matches did the team play, and how many goals did it score in all?
10 matches, because 1 + 3 + 4 + 1 + 1 = 10. Goals: 0 + 3 + 8 + 3 + 4 = 18 goals.
3
How many more matches had 1 goal than matches with 0 goals?
2 more matches: 3 − 1 = 2.
4
In Mostafa’s dot plot, 2 bags had 10 beans. If we pour those 2 bags onto the table, how many beans are there?
10 + 10 = 20 beans.
5
The data are 5, 6, 6, 7, 7, 7, 8. How many dots go above 7, and how many dots are there in all?
3 dots above 7. There are 7 dots in all, one for each piece of data.