25.11.2014 Views

Biostatistics

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

2.5 DESCRIPTIVE STATISTICS: MEASURES OF DISPERSION 43<br />

the skewed distributions in a similar fashion. Using SPSS software, the following<br />

descriptive statistics were obtained for these three distributions<br />

No Skew Right Skew Left Skew<br />

Mean 8.0000 6.6667 8.3333<br />

Median 8.0000 6.0000 9.0000<br />

Mode 8.00 5.00 10.00<br />

Skewness .000 .627 .627<br />

&<br />

2.5 DESCRIPTIVE STATISTICS:<br />

MEASURES OF DISPERSION<br />

The dispersion of a set of observations refers to the variety that they exhibit. A measure of<br />

dispersion conveys information regarding the amount of variability present in a set of data.<br />

If all the values are the same, there is no dispersion; if they are not all the same, dispersion is<br />

present in the data. The amount of dispersion may be small when the values, though<br />

different, are close together. Figure 2.5.1 shows the frequency polygons for two populations<br />

that have equal means but different amounts of variability. Population B, which is<br />

more variable than population A, is more spread out. If the values are widely scattered, the<br />

dispersion is greater. Other terms used synonymously with dispersion include variation,<br />

spread, and scatter.<br />

The Range One way to measure the variation in a set of values is to compute the<br />

range. The range is the difference between the largest and smallest value in a set of<br />

observations. If we denote the range by R, the largest value by x L , and the smallest value<br />

by x S , we compute the range as follows:<br />

R ¼ x L x S (2.5.1)<br />

Population A<br />

Population B<br />

FIGURE 2.5.1<br />

of dispersion.<br />

m<br />

Two frequency distributions with equal means but different amounts

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!