## Lecture

- Measures of Variability
- Index of Qualitative Variability – used for nominal data
- The range – used for interval data – simply the largest value minus the lowest value
- The interquartile range, the difference between the cutpoint where 25 percent of the cases have a larger values, and the cutpoint where 25 percent of the cases have a lower value
- The boxplot shows the range, the interquartile range and the median

- The variance and standard deviation, a measure of the amount scores cluster at the mean or spread out from the mean. Used for interval data. The variance is the average squared deviation of scores from the mean
- The standard deviation is calculated from the variance. You simply take the square root of the variance.
- The standard deviation is calculated from the variance. You simply take the square root of the variance.

- The normal curve
- Symmetric around the middle
- Mean, median and mode are the same

- The concept of Z scores
- Used to find area under the normal curve
- Equals the number of standard deviations any score is from the mean
- Calculations – subtract the mean from the score and divide by the standard deviation
- Finding the percent of cases – the same as the percent area – between any point and the mean
- Using the z table
- Going from percents to z scores

## Homework

Assignment, Chapter 4 Exercise 2, Chapter 5 Exercises 2,4