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Chapter 3

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University of Toronto St. George

Psychology

PSY201H1

Kristie Dukewich

Fall

Description

Understanding Statistics Chapter 3
PSY201
Grouping Scores
Frequency distribution of grouped scores
How scores are to be presented when there are many so that we can see the shape
of the distribution
To construct a frequency distribution of grouped scores
Find the range of scores
Highest sore minus the lowest score
Determine the width of each class interval (i)
i= range/# of class intervals
Remember to count EACH unit, so 50-55 isn’t 5 units wide, it’s 6 units wide
List the limits of each class interval, placing the interval containing the lowest
score value at the bottom
Start with the lowest, which must contain the lower limit of the interval
Make the lower limit divisible by i! So if the lowest unit is 42, start with 40 if
the i is 5 or 10
Tally the raw scores into the appropriate class intervals
Add the tallies of each interval to obtain the interval frequency
Relative Frequency, Cumulative Frequency, and Cumulative Percentage Distributions
Relative frequency distribution
Indicates the proportion of the total number of scores that occurs in each interval
Relative frequency = f/N [frequency divided by total number of scores]
Cumulative frequency distribution
Indicates the number of scores that fall below the upper limit of each interval
Add the frequency of that interval to the frequencies of all the intervals below it
Cumulative percentage distribution
Indicates the percentage of scores that fall below the upper real limit of each
interval
Convert cumulative frequencies to cumulative percentage
cum%=[(cumf)/N] X100
Percentiles
A percentile, or percentile point, is the value on the measures scale below which a
specified percentage of scores in the distribution fall
A measurement of relative standing, and to compare an individual score to that of a
reference group
th
Notation: for 50 percentile point, we write P 50 Means: Value below which 50% of the other values fall
To find a percentile point
Determine the frequency of the scores below the percentile

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