Textbook Notes (280,000)
US (110,000)
WUSTL (200)
Chapter 4

# L33 Psych 300 Chapter 4: Central Tendency and Variability

Department
Psychological & Brain Sci (Psychology)
Course Code
L33 Psych 300
Professor
Nestojko
Chapter
4

This preview shows half of the first page. to view the full 1 pages of the document. Chapter 04: Central Tendency and Variability
Central Tendency - the descriptive statistic that best represents the center of the data set, the
particular value that all the other data seems to be gathering around
Mean, the Arithmetic Average
Mean in Plain English - the arithmetic average of a group of scores (often just
called the average), a precise number that should be the “typical” score in a
distribution
Mean in Plain Arithmetic - calculated by summing all the scores in a data set and
dividing this sum by the total number of scores
Mean Expressed by Symbolic Notation - M
or “X-bar” (X with a bar above)
The numbers based on samples taken from a population are called
statistics; M is a statistic. The numbers based on whole populations are
called parameters, “U” is a parameter. “U” is pronounced “Mew”
How to calculate the mean: add up all the scores and divide by the # of scores
Median - the middle score of all the scores in a sample when the scores are arranged in
ascending order
With an odd number of scores, there will be an actual middle score
With an even number of scores, there will be no actual middle, so you have to
calculate the mean (average) of the two middle scores
Mode - the most common score of all the scores in a sample
When there is no specific mode, we can report the most common interval as the
mode; for example, we would say that the mode on a test is 80-89
Unimodal - when a distribution of scores has one mode
Bimodal - when a distribution of scores has two modes
Multimodal - when a distribution of scores has more than two modes
Measures of Variability
Variability - a numerical way of describing how much spread there is in a distribution
Numerically - range
Computing variance and its square root, known as standard deviation
Range - a measure of variability calculated by subtracting the lowest score (minimum)
from the highest score (maximum)
Range = X(highest) - X(lowest)
Variance - the average of the standard deviations from the mean
How do we calculate the variance?
Subtract the mean from every score
Square every deviation from the mean
Sum all of the squared deviations
Divide the sum of squares by the total number in the sample (N)
Standard Deviation - the square root of the average of the squared deviations from
the mean, or more simply, the square root of the variance