Thursday, October 13, 2011
Statistics
Distributions
Central Tendency
Variability
Distributions
Simplify with a few numbers (Statistics)
Mean (); M
Add up numbers & divide by
N
(the number of students)
(xmas) = 60.69
average grade for the Christmas exam
Median (MDN)
Midpoint of ranked distribution
List of numbers, find median. Rank from low to high.
MDN (xmas) = 54
Note: N = 1135
Look at the score in the middle, i.e., score #568
(This leaves 567 scores on each “side”)
Mode
Most frequent score
Easy to identify from frequency distribution
Mode xmas = 54
(Frequency = 47)
Note:
≈ MDN ≈ MODE
Shape (of frequency distribution curve)
Overall shape determined by dispersion of scores…
1
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How do they cluster around the mean?
Range:
Difference between highest and lowest scores.
XMAS
98 – 24 = 74
Note:
1.
Range influenced by extreme scores
2.
Same value can be obtained with a variety of scores, e.g., 84 – 10 – 74
Variance (s
2
)
Degree of fluctuation of scores around the mean
Some distributions have high variance, others are relatively low
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 Fall '09
 ATKINSON
 Psychology, Statistics, Normal Distribution, Standard Deviation, Statistical hypothesis testing, Statistical significance, statistically significant result

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