Data
Charts & tables
A chart makes a pattern jump out of a list of numbers. But read the scale first — two charts can look identical and mean completely different things.
Reading a bar chart
Each bar's height shows how much or how many. To read one: 1. Find the top of the bar. 2. Read across to the scale. 3. Check the units on the axis label. If the top falls between two marks, work out what each small step is worth first.
Always check the scale
Two bar charts can look almost the same and show completely different amounts. Class A recorded litter in KILOGRAMS, going up to 2 kg. Class B recorded it in GRAMS, going up to 1000 g. A bar that reaches the top means 2 kg on one chart and 1 kg on the other. You cannot compare them by eye — you must read the scale and the units. This is the single commonest way charts mislead people.
Dot plots
A DOT PLOT puts one dot for each item above its value on a line. Counting the dots in a column tells you the frequency. Where the dots bunch up tells you where most of the data lies. If most dots sit above ages 2, 3 and 4, then most of the children were that age — which is exactly the kind of thing a zoo would want to know before building a play area.
Using data to answer a question
A chart is only useful if it helps you decide something. Look for: the biggest and smallest values, where the data bunches, and anything that looks out of place. Then say what it means. 'The play area should be aimed at 2 to 4 year olds, because most of the children at the zoo were that age' — a conclusion supported by the data, not just a number read off.
Worked examples
Reading a value from a dot plot
- Find the column above age 2.
- Count the dots in that column.
Answer: 13 children were 2 years old
Adding two columns
- Children over 9 means the columns to the right of 9.
- Count the dots in each and add them.
Answer: 2 children were over 9
Comparing charts with different scales
- Class A's axis is in kilograms up to 2.
- Class B's axis is in grams up to 1000.
- 1000 g = 1 kg, so the same bar height means different amounts.
Answer: You must convert to the same unit before comparing
Converting for a fair comparison
- Class B collected 800 g in week 3.
- 1000 g = 1 kg, so 800 g = 0.8 kg.
- Now it can be compared with Class A's kilograms.
Answer: 800 g = 0.8 kg
Drawing a conclusion
- Most dots sit above ages 2, 3 and 4.
- So most visitors are young children.
Answer: Aim the play area at 2 to 4 year olds
Where you see this in real life
- A shop's sales chart shows which day was busiest at a glance.
- Two newspapers can make the same figures look very different by changing the scale.
- A doctor plots a child's height each year to see the pattern of growth.
Key words
- Bar chart
- A chart using bars whose heights show amounts.
- Dot plot
- A chart with one dot for each item above its value.
- Scale
- The numbers marked along an axis.
- Axis
- A labelled line along the side or bottom of a chart.
- Frequency
- How many times something occurs.
- Data
- The information you collect.