Frequency Distributions

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# Frequency Distributions - PowerPoint PPT Presentation

Frequency Distributions. PSY 211 8-29-07. Frequency Distribution. Definition: Table/graph showing number of participants in each category. If I gave a 5-point quiz to a class, the distribution of scores might look like this:.

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## Frequency Distributions

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Presentation Transcript
Frequency Distribution
• Definition: Table/graph showing number of participants in each category
If the scores are spread out, it is okay to group them into a range of scores

Some data:

96, 68, 74, 82, 86,

51, 66, 82, 91, 85,

77,89,88,88,61,90,

79,86,80,92,88,69,

44,99,81,82

Polygon of Quiz Scores

f

6

Scores

%

Scores

Distribution Shape
• Rather than graph an entire distribution, sometimes researchers just describe its shape and provide a few statistics

f

x

f

Symmetrical distributions: look the same on both sides, mirror images

x

f

x

Positive Skew

*Skewed distribution: lopsided, have a tail off to one side

Tail indicates the direction of the skew (+ or -)

f

x

Negative Skew

f

x

Negative Skew

In actuality, frequency polygons look more like these

f

x

f

Symmetrical

x

Percentile Rank
• Used to describe where a score falls in a given distribution

95th percentile

50th percentile

f

f

x

x

Finding Percentile Ranks

cf = cumulative frequency, number of people with that score or lower

Finding Percentile Ranks

c% = cumulative percentage or percentile rank, percent with that score or lower