Bar charts (including multiple and composite), line graphs, frequency polygons, population pyramids and choropleth maps; representing, interpreting and comparing data sets shown graphically.

What this dot point is asking

Edexcel codes 2a.02 and 2a.03 require you to represent, interpret and compare data sets displayed graphically. The graph types are bar charts (including multiple and composite, both ordinary and percentage), line graphs, frequency polygons, population pyramids and choropleth maps. (Time series, scatter diagrams, cumulative frequency, histograms and box plots are covered on their own pages.) You must read values off accurately, calculate from percentages, and make clear comparisons between data sets in context.

Bar charts

A bar chart displays the frequency of categories using bars of equal width separated by gaps (the gaps distinguish it from a histogram, where bars touch). Variations Edexcel expects:

• Multiple (grouped) bar chart. Bars for two or more groups stand side by side for each category, so you can compare, for example, male and female frequencies category by category.
• Composite (stacked) bar chart. Components are stacked within a single bar, showing both the total and the split. A percentage composite bar chart scales every bar to $100\%$, which compares the proportions across groups of different totals.

To read a stacked bar, the length of a segment gives that component; to compare percentage composites, focus on the relative sizes of the segments rather than absolute lengths.

Line graphs

A line graph joins plotted points with straight lines to show how a quantity changes, usually over time. Line graphs suit continuous change and trends; reading between plotted points is interpolation, and you should be cautious extending a line beyond the data (extrapolation).

Frequency polygons

A frequency polygon is drawn by plotting the frequency against the midpoint of each class interval and joining the points with straight lines. It shows the shape of a grouped distribution and is especially useful for comparing two distributions on the same axes (for example test scores of two classes). Edexcel notes that frequency polygons may be open or closed; the key skill is using class midpoints, not class boundaries.

Population pyramids and choropleth maps

A population pyramid is a back-to-back horizontal bar chart showing the age structure of a population, usually split by sex, with age bands up the middle. A wide base means many young people (a growing population); a wide top means many older people (an ageing population). A choropleth map shades or colours geographical regions according to the value of a variable (for example population density), with darker shades for higher values; a clear key is essential, and you interpret patterns across regions.

Choosing and justifying a chart

Edexcel frequently asks you to select and justify an appropriate chart for a given data set or audience. Match the chart to the data and purpose:

• Bar chart for comparing the frequencies of categories.
• Multiple bar chart for comparing two or more groups across the same categories.
• Composite (percentage) bar chart for showing how a total splits into parts, especially when comparing the make-up of groups of different sizes.
• Line graph for showing change in a continuous quantity over time.
• Frequency polygon for comparing the shapes of two grouped distributions.
• Population pyramid for age and sex structure; choropleth map for values that vary by region.

A good justification names both the data type and the purpose: for example, "a percentage composite bar chart, because we are comparing how the travel split changes between two years with different totals". Consider the audience too, since a simple chart often communicates more clearly than a technical one.

Reading values accurately

Many marks here are simply for reading a graph carefully. Always check the scale (how much each gridline is worth), read to the nearest sensible value, and watch for a vertical axis that does not start at zero, which exaggerates differences. When a question asks you to calculate from a chart (for example a percentage of a total, or the difference between two groups), set out the arithmetic clearly so method marks are secure even if a reading is slightly off.