27.03.2013 Views

Brian S. Everitt A Handbook of Statistical Analyses using SPSS

Brian S. Everitt A Handbook of Statistical Analyses using SPSS

Brian S. Everitt A Handbook of Statistical Analyses using SPSS

SHOW MORE
SHOW LESS

Create successful ePaper yourself

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

lifespan in days<br />

diet<br />

Restricted diet<br />

Ad libitum diet<br />

Descriptives<br />

Mean<br />

95% Confidence<br />

Interval for Mean<br />

5% Trimmed Mean<br />

Median<br />

Variance<br />

Std. Deviation<br />

Minimum<br />

Maximum<br />

Range<br />

Interquartile Range<br />

Skewness<br />

Kurtosis<br />

Mean<br />

95% Confidence<br />

Interval for Mean<br />

5% Trimmed Mean<br />

Median<br />

Variance<br />

Std. Deviation<br />

Minimum<br />

Maximum<br />

Range<br />

Interquartile Range<br />

Skewness<br />

Kurtosis<br />

Display 2.3 Descriptives output for rat data.<br />

Lower Bound<br />

Upper Bound<br />

Lower Bound<br />

Upper Bound<br />

Statistic Std. Error<br />

968.75<br />

913.94<br />

27.641<br />

1023.55<br />

988.31<br />

1035.50<br />

80985.696<br />

284.580<br />

105<br />

1435<br />

1330<br />

311.50<br />

-1.161 .235<br />

1.021 .465<br />

684.01 14.213<br />

655.77<br />

712.26<br />

695.05<br />

710.00<br />

17978.579<br />

134.084<br />

89<br />

963<br />

874<br />

121.00<br />

-2.010 .255<br />

7.027 .506<br />

group). Other measures <strong>of</strong> spread, such as the standard deviation and the<br />

range <strong>of</strong> the sample, confirm the increased variability in the restricted diet<br />

group.<br />

Finally, <strong>SPSS</strong> provides measures <strong>of</strong> two aspects <strong>of</strong> the “shape” <strong>of</strong> the<br />

lifespan distributions in each dietary group, namely, skewness and kurtosis<br />

(see <strong>Everitt</strong> and Wykes, 1999). The index <strong>of</strong> skewness takes the value<br />

zero for a symmetrical distribution. A negative value indicates a negatively<br />

skewed distribution, a positive value a positively skewed distribution —<br />

Figure 2.1 shows an example <strong>of</strong> each type. The kurtosis index measures<br />

the extent to which the peak <strong>of</strong> a unimodal frequency distribution departs<br />

from the shape <strong>of</strong> normal distribution. A value <strong>of</strong> zero corresponds to a<br />

normal distribution; positive values indicate a distribution that is more<br />

pointed than a normal distribution and a negative value a flatter distribution<br />

— Figure 2.2 shows examples <strong>of</strong> each type. For our data we find<br />

that the two shape indices indicate some degree <strong>of</strong> negative skewness<br />

© 2004 by Chapman & Hall/CRC Press LLC

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

Saved successfully!

Ooh no, something went wrong!