Frequency Distributions

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Chapter 2

• Frequency Distributions

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Short term memory demonstration
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How can we make sense out of the data?

• Place the scores in ascending or descending order
• Count the frequency of each score (i.e.,
  construct a frequency distribution)
• Other?

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Relative Frequency
Reporting raw frequency can be somewhat
misleading since it doesnt take into account the
size of the sample (n). Converting raw
frequency to relative frequency avoids that
problem.
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Percentile Rank
Percentile rank is the number of scores equal to
or less than a particular value. Percentile rank
allows us to evaluate a particular score.
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Often percentile rank is defined as the
percentage of scores less than a particular
value. The difference in the definition has
little affect when dealing with an entire
population, it does however when dealing with a
sample.
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Constructing Frequency Polygons
Relative Frequency
X values
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Example using short-term memory scores
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Interpreting Frequency Polygons
The characteristics of a distribution of scores
can easily be seen in a frequency polygon.
Consider the following polygons. How does each
pair differ?
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These distributions differ with regard to the
magnitude of the scores. This can easily be seen
by comparing their center points.
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How do these distributions differ?
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How do these distributions differ?
These distributions differ with regard to their
variability, the degree to which scores differ.
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How do these distributions differ from the
preceding distributions?
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How do these distributions differ from the
preceding distributions?
These distributions are skewed. The previous
distributions were symmetrical.
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Some distributions are positively skewed. Others
are negatively skewed.
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Describe this distribution
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Describe this distribution
This is a bimodal distribution. It has two modes.
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Is this a bimodal distribution?
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How about this one?
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How about this one?
Whether a researcher refers to a distribution as
bimodal depends on whether the researcher
believes the secondary peak is important in the
context of the research.
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What can we say about the distribution of
short-term memory scores based on its frequency
polygon?