Bar and Pie Charts
Bar and pie charts use pictures to compare the sizes, amounts, quantities, or proportions of various items or groupings of items.
When to Use Bar and Pie Charts
Bar and pie charts can be used in defining or choosing problems to work on, analyzing problems, verifying causes, or judging solutions. They make it easier to understand data because they visually present data, highlighting the results in ways that may be easier for team members, managers, and other stakeholders to understand.
Bar and pie charts present results that compare different groups. They can also be used with variable data that have been grouped. Bar charts work best when showing comparisons among categories. Pie charts can be used when you want to compare parts to a whole. Pie charts are used for showing relative proportions of various items in making up the whole (how the "pie" is divided up).
Selecting a Type of Bar Chart
Teams may choose from three types of bar charts, depending on the type of data they have and what they want to stress:
Simple bar charts sort data into simple categories.
Grouped bar charts divide data into groups within each category and show comparisons between individual groups as well as between categories. (It gives more useful information than a simple total of all the components.)
Stacked bar charts, which, like grouped bar charts, use grouped data within categories, except that the bars representing the subgroups are placed on top of each other to make a single column, or side by side to make a single bar. They make clear both the sum of the parts and each group’s contribution to that total. Stacked bar charts that sum to 100% can also be used to show the contribution of parts to a whole, similar to pie charts.
How to Use a Bar Chart
Step 1. Choose the type of bar chart that stresses the results you want people to focus on. Grouped and stacked bar charts will require at least two classification variables (for example, Performance of sites on MOH quality standards as “good”, “fair”, or “poor”). For a stacked bar chart, the data within each category are tallied into combined totals before creating the chart.
Step 2. The vertical axis represents the values of the variable of comparison (e.g., number, percent).
Step 3. The number of bars you will use will equal the number of categories for simple or stacked bar charts. For a grouped bar chart, the number of bars will equal the number of categories multiplied by the number of groups.
Step 4. Each item’s bars should be of equal width. Be sure to label each category on the horizontal axis, and if applicable, the groups. Provide a title for the graph that indicates the sample and the time period covered by the data.
How to Use a Pie Chart
Step 1. Create a pie chart only if the sum of all categories is meaningful (for example for proportions). Each category will represent the contribution of each category to the whole (100%).
Step 2. Provide a title for the pie chart that indicates the sample and the time period covered by the data. Label each segment with its percentage or proportion (e.g., 25%, 75% etc.) and with what each segment represents (e.g., people who returned for a follow-up visit; people who did not return). Remember, in a pie chart, all parts need to be mutually exclusive, with no overlap between parts. All parts need to add up to 100%. If there are more than five to seven parts, or if you want to compare parts to each other rather than parts to a whole, use a different type of graph.
Points to Remember
Be careful not to use too many notations on the charts. Keep them as simple as possible and include only the information necessary to interpret the chart.
Do not draw conclusions not justified by the data. For example, determining whether a trend exists may require more statistical tests and probably cannot be determined by the chart alone. Differences among groups also may require more statistical testing to determine if they are significant.
Whenever possible, use bar or pie charts to support data interpretation. Do not assume that results or points are so clear and obvious that a chart is not needed for clarity.
To ensure that charts do not mislead, follow these guidelines:
- Scales must be in regular intervals
- Charts that are to be compared must have the same scale and symbols
- Charts should be easy to read