| Scales of Measurement:
Nominal Scale: A set of categories for classifying people or
objects (e.g., eye color, gender, political affiliation)
Ordinal Scale: A scale indicating the order or relative position
of items or people based on some criterion (i.e., 1st place, 2nd, 3rd,
Interval Scale: Scale with equal distances between points, but
with no true zero point (e.g., temperature, most psychological tests)
Ratio Scale: Scale with equal distances between points and with a
true zero point, thus we can say something is twice as much as something
else (e.g., inches of rainfall, distance in miles, etc.)
Measures of Central Tendency
Mean: Arithmetical average calculated by dividing a sum of
values by the total number of cases
Median: Point that divides a set of scores in half.
Mode: The most frequent score in a distribution of scores
*Of these three measures, the MEAN is most affected by outliers
or extreme scores.
Measures of Variation
Range: Difference between the largest and smallest scores in a
Variance: A statistical average of the amount of dispersion
around the mean in a distribution of the scores. It is the
Standard Deviation squared.
Standard Deviation: A statistical measure of the amount of
dispersion in a set of scores. Specifically, it is the square root
of the average squared deviations from the mean of a set of scores.
It is simply the square root of the variance.
*Of the three measures, the STANDARD DEVIATION is most affected by
Distributions of Scores
Normal Curve: Hypothetical, bell-shaped distribution of scores
that occurs when a normal distribution is plotted as a frequency
In a normal distribution, the mean, median, and mode are all equal
and divide the distribution in half (the 50th percentile).
Percentile Rank: Reflects the percentage of subjects who score lower
than the subject in question
|The Normal Distribution
Positively Skewed Distribution: A distribution where most scores
are clustered at the lower end of the curve, with a few very high scores
creating a long "tail" to the right. The mean is
greater than the median, and the median is greater than the mode.
Negatively Skewed Distribution: A distribution where most scores
are clustered at the upper end of the curve, with a few very low scores
creating a long "tail" to the left. The mean is less
than the median, and the median is less than the mode.