Multiple and composite bar charts (Edexcel GCSE Statistics): Revision Notes
Multiple and composite bar charts
Bar charts are powerful tools for displaying data, and there are two special types that you need to understand for your GCSE exams: multiple bar charts and composite bar charts. Both help us compare different groups and categories, but they work in different ways.
These chart types are frequently tested in GCSE exams because they require you to interpret data carefully and demonstrate your understanding of how different visual representations convey information.
What are multiple bar charts?
Multiple bar charts display more than one bar for each category or group. Think of them as showing several different data sets side by side, making it easy to compare values across different groups.
Key Features of Multiple Bar Charts:
- Several bars grouped together for each category
- Each bar represents a different subcategory or time period
- All bars use the same scale on the vertical axis
- The bars must appear in the same order for each main category
Reading multiple bar charts
Let's look at how multiple bar charts work using a car sales example. Imagine you're looking at a chart showing car sales for three different manufacturers (Honda, Ford, and Seat) across four quarters of a year.
In this type of chart, you'll see groups of three bars for each quarter. Each group contains one bar for Honda (shown in yellow), one for Ford (shown in blue), and one for Seat (shown in red). The height of each bar tells you how many cars each manufacturer sold in that particular quarter.
This layout makes it incredibly easy to spot patterns. For instance, you might notice that Seat's sales were particularly high in the first two quarters but then declined in the later quarters. You can also quickly compare how each manufacturer performed relative to the others in any given quarter.
What are composite bar charts?
Composite bar charts (sometimes called compound bar charts) work differently from multiple bar charts. Instead of placing bars side by side, they stack different categories within a single bar. These charts often display information as percentages, showing how different parts make up the whole.
Main Characteristics of Composite Bar Charts:
- Each bar is divided into segments representing different subcategories
- The segments are stacked on top of each other
- The total height of each bar represents 100% (or the total value)
- Different colours represent different categories
- A key or legend shows what each colour represents
Understanding composite bar charts
Consider an example showing the percentages of men and women seen jogging over two different years. In this chart, each bar represents one year, and each bar is divided into two coloured segments - one for men (blue) and one for women (pink).
The chart might show that in 2010, about 75% of joggers observed were men and 25% were women, whilst in 2012, this had changed to about 60% men and 40% women. This tells us that the proportion of female joggers increased significantly between these two years.
Composite bar charts excel at showing how proportions within groups change over time or differ between categories. However, they can be trickier to read than multiple bar charts because you need to carefully identify where each segment begins and ends.
Worked example: reading composite bar charts
Let's work through a specific example to master the technique of reading composite bar charts accurately.
Worked Example: Reading Composite Bar Chart Data
A composite bar chart shows the percentages of families with 1, 2, or 3+ children in a town during 1980 and 2010. Did the percentage of families with 2 children increase or decrease between 1980 and 2010?
Step 1: Identify what each segment represents Look at the key to understand that:
- Light grey = 1 child
- Medium grey = 2 children
- Dark grey = 3 or more children
Step 2: Find the relevant segments We need to focus on the medium grey sections (representing families with 2 children) in both the 1980 and 2010 bars.
Step 3: Read the values carefully Here's the crucial technique - when reading composite bar charts, you must identify the top and bottom boundaries of the segment you're interested in:
For 1980:
- The "2 children" segment starts at 40% and ends at 80%
- So families with 2 children = 80% - 40% = 40%
For 2010:
- The "2 children" segment starts at 48% and ends at 84%
- So families with 2 children = 84% - 48% = 36%
Step 4: Compare and conclude Since 40% > 36%, the percentage of families with 2 children decreased between 1980 and 2010.
Essential tips for reading composite bar charts
Critical Reading Techniques:
Always check if the chart starts at zero: This is absolutely critical. If a segment doesn't begin at the bottom of the chart (at 0%), you cannot simply read its value from the top. You must subtract the bottom value from the top value to get the correct percentage or amount.
Use the key systematically: Always refer back to the key to ensure you're reading the correct category. It's easy to mix up similar colours, especially under exam pressure.
Read both endpoints: For any segment in a composite bar chart, identify both where it starts and where it ends, then calculate the difference.
Double-check your arithmetic: Simple subtraction errors are common in exams, so always verify your calculations.
Look for patterns: Once you've extracted the data, step back and consider what the overall trend tells you. Are values increasing, decreasing, or staying roughly constant?
Key Points to Remember:
- Multiple bar charts show several bars side by side for each category, making comparisons between groups straightforward
- Composite bar charts stack segments within single bars to show proportions and totals, perfect for displaying percentages
- When reading composite bar charts, always subtract the bottom value from the top value of your target segment - never just read the top value alone
- Both chart types require a clear key or legend to interpret the data correctly
- Multiple bar charts are generally easier to read quickly, whilst composite bar charts are excellent for showing how parts relate to the whole