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Lecture 4
Psyc 300A
Measures of Central Tendency
• Mode: The most frequently occurring
score
• Median: The midpoint in an ordered
set of scores. The 50th percentile.
• Mean: Sum of all scores divided by the
number of scores.
• Which measure to use?
Measures of Dispersion
• Range: Highest score minus lowest
score.
• Standard Deviation: Average amount
of variability in a set of scores.
Technically, the square root of variance.
• Variance: Average squared deviation
from the mean.
Formulas to Calculate the SD
Standard Deviation
X
_
(X – X)
_
(X – X)2
8
8
8
7
6
6
5
5
4
3
+2
+2
+2
+1
0
0
-1
-1
-2
-3
4
4
4
1
0
0
1
1
4
9
sum
0
28
s = Sqrt (28/n-1)= 1.76
Using SPSS
• What is SPSS?
• Setting up a data file
• Running a frequency analysis with
descriptive statistics