1. Wednesday, September 26
Appreciating the beautiful
world of data…

2. …using your eyeballs and your brain

4. Types of data Nominal Ordinal Interval/Ratio
Think of these in terms of information value!

5. Current NL West Baseball Standings

6. Looking at Distributions
Frequency distributions
Stem-and-Leaf Display

7. Central tendency

8. The mode is the score with the highest frequency of occurrences.
It is the easiest score to spot in a distribution.
It is the only way to express the central tendency of a nominal
level variable.

9. The median.
The median is the middle-ranked score (50th percentile).
If there is an even number of scores, it is the arithmetic
average of the two middle scores.
The median is unchanged by outliers. Even if Bill Gates
were deleted from the U.S. economy, the median asset of
U.S. citizens would remain (more or less) the same.

10.  The Mean The mean is the arithmetic average of the scores. Xi _
_________ i X = N

11.  The Mean The mean is the arithmetic average of the scores.
The mean is the center of gravity of a distribution. Deleting
Bill Gates’ assets would change the national mean income.  Xi _
_________ i X = N

12. The mean of a group of scores is that point on the number line
such that the sum of the squared distances of all scores to that point
is smaller than the sum of the squared distances to any other point.

13. The Mean
The sum of squared deviations from the Mean is at the lowest value. _ ( ) 2  Xi - X
is lowest

14. The Mean
The sum of squared deviations from the Mean is at the lowest value. _ ( ) 2  Xi - X
is lowest _ X

15. The Mean
The mean is the arithmetic average of the scores.
The mean is the center of gravity of a distribution. Deleting
Bill Gates’ assets would change the national mean!
The sum of squared deviations from the Mean is at
the lowest value.
The mean is not a good measure of central tendency if there
are outliers.

16. Variability

17. For the eyeball: Range, Interquartile Range
Range: Highest minus lowest score.
Interquartile Range: 75th percentile score minus 25th percentile score.

18. SS Variance of a population, 2 (sigma squared).
It is the sum of squares divided (SS) by N SS 2 = N

19. SS Variance of a population, 2 (sigma squared).
It is the sum of squares divided (SS) by N
 (X –  ) 2 SS 2 = N

20. The Standard Deviation of a population, 
It is the square root of the variance. SS  = N
This enables the variability to be expressed in the same unit of measurement
as the individual scores and the mean.