Pie chart

From Wikipedia, the free encyclopedia

A pie chart is a circular chart divided into sectors, illustrating relative magnitudes or frequencies. In a pie chart, the arc length of each sector (and consequently its central angle and area), is proportional to the quantity it represents. Together, the sectors create a full disk. A chart with one or more sectors separated from the rest of the disk is called an exploded pie chart.

The earliest known pie chart is generally credited to William Playfair's Statistical Breviary of 1801.

1 Polar area diagram
2 Example
3 Warning against usage
4 See also
5 References
6 External links

[edit] Polar area diagram

Florence Nightingale is credited with developing an early form of the pie chart which she called the "coxcomb" which she first published in 1858. This form of pie chart is now known as the polar area diagram (Polar-Area Diagram), or occasionally the Nightingale rose diagram. The polar area diagram is similar to a usual pie chart, except that the sectors are each of an equal angle and differ rather in how far each sectors extends from the centre of the circle. It has been suggested that most of Nightingale's early reputation was built on her ability to give clear and concise presentations of data.

[edit] Example

A pie chart for the example data.

An exploded pie chart for the example data, with the largest party group exploded.

The following example chart is based on the results of the election for the European Parliament in 2004. The following table lists the number of seats allocated to each party group, along with the percentage of the total that they each make up. The values in the last column, the central angle of each sector, is found by multiplying the percentage by 360°.

Group	Seats	Percent (%)	Central angle (°)
EUL	39	5.3	19.2
PES	200	27.3	98.4
EFA	42	5.7	20.7
EDD	15	2.0	7.4
ELDR	67	9.2	33.0
EPP	276	37.7	135.7
UEN	27	3.7	13.3
Other	66	9.0	32.5

[edit] Warning against usage

Pie charts are rare in the scientific literature, but are more common in business and economics. One reason for this may be that it is more difficult for comparisons to be made between items in a chart when area is used instead of length. In Stevens' power law, visual area is perceived with a power of 0.7 compared to length that is 1.0. This implies that length would be a better scale to use, since differences would be linearly related.

In research at AT&T Bell Laboratories, it was shown that comparisons using angles was less accurate than comparisons using length. This can be illustrated with the diagram below. Most subjects have difficulty ordering the slices in the pie chart, however when a bar chart was used the comparison is much clearer. ^[1]