A picture of the numbers

The idea

A dot plot shows each data value as a dot on an equally spaced line, stacking repeated values; it loses the data's original order but shows its extremes and how it clusters or spreads out.

Heights as a list

The heights of 11 girls in a Grade 5 class were noted in this order: 143, 136, 150, 144, 154, 140, 145, 148, 156, 150, 150.

How quickly can you see the tallest, the shortest and where most girls are?

• At once, from the list
• Not at once: I would need to look at all the numbers
• A list can never show this

Drop the dots

Tap each number. Its dot drops onto the height line. If two girls have the same height, the dots stack.

A dot plot

A dot plot shows each value as a dot on a line with equal spacing. The three girls of height 150 cm make a stack of three dots.

Now the ends are easy to see: the shortest girl is 136 cm and the tallest is 156 cm. We also see where the dots cluster and where they are spread out. But we lost something: the plot does not show which girl was measured first.

A dot plot shows each data value as a dot on an equally spaced line, stacking repeated values. It loses the data’s original order but shows its extremes and how it clusters or spreads out.

Notes

A dot plot shows each data value as a dot on an equally spaced line, stacking repeated values. It loses the data’s original order but shows its extremes and how it clusters or spreads out.

Check yourself

In the dot plot, three dots are stacked above 150. What does it mean?

Which of these can you not find out from a dot plot alone?

How many girls have a height from 143 cm to 148 cm (both included)? Count the dots in the picture.

Answer: 4

The dots are at 143, 144, 145 and 148: 4 girls.

How many girls are taller than 148 cm? Count the dots to the right of 148.

Answer: 5

Three dots at 150, one at 154 and one at 156: 3 + 1 + 1 = 5 girls.

• One girl is 3 × 150 cm tall.. Each dot is one girl. The stack counts how many girls have that height.
• The 150th girl is the tallest.. The number on the line is the height, not the position of a girl.
• Three girls are 150 cm tall. — correct. Yes! Each dot is one value, and equal values stack.
• Which girl was measured first — correct. Yes! A dot plot sorts the values, so the original order is lost.
• The smallest and the biggest value. These are the leftmost and rightmost dots. A dot plot shows them well.
• Where most of the values cluster. Dots that pile up show the cluster. A dot plot shows this well.