Interpreting Bar Charts and Pie Charts • In this presentation • Bar charts • Pie Charts • Interpretation Guidelines
Bar Charts • Bar Charts are used to display qualitative data • Bar charts may display: • Counts (or frequencies) • Percentages • Proportions • This allows comparison between categories.
Bar Charts • The vertical scale is the frequency, relative frequency, or percentage. • This is critically important to recognize and should be labeled. • Frequencies are raw counts. That is, how many individuals prefer each of the colors. • Relative frequencies are really proportions (usually decimals). To find the relative frequencies, divide the frequency by the total (for red, 11/25 or .44). • Percentages are relative frequencies expressed as percentages (for red, 44%). • See the example.
Bar Charts • The horizontal scale shows the different categories (sometimes multiple categories are lumped together into an “other” category).. • All bars should have the same width. • There are gaps between the bars (since there is no connection between them) • Categories may be listed in any order. • More complicated bar charts exist (such as stacked or side-by-side).
Bar Chart Example Back to slide 4 on Bar Charts
Pie Charts • A circle is divided up proportionately to show what percentage of the total each category represents. • They convey information regarding only the relative size of categories (because of this, they may mislead as totals remain unknown).
Interpretation Guidelines • Carefully read all labels (title, axes, bars, pie segments, etc.) including the units and scale (be sure to note the scale’s starting point). • Identify (if possible) the source of the data. Is there sufficient reason to suspect the source may be attempting to persuade? In what way?
Interpretation Guidelines • Consider and watch for extreme values or other possible anomalies. • Do the bars or pie sections represent counts or relative counts? • If there are multiple displays, are their scales and axes consistent?
Interpreting Bar Charts and Pie Charts • This concludes the presentation