HISTOGRAMS

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HISTOGRAMS

• Representing Data
• OCR Module 9

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Why use a Histogram

• When there is a lot of data
• When data is
• Continuous
• a mass, height, volume, time etc
• Presented in a Grouped Frequency Distribution
• usually in groups or classes that are UNEQUAL

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Continuous data
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Determined by Grouped Frequency Distribution
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Frequency Class width
NOT height, because of UNEQUAL classes!
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Grouped Frequency Distribution
Classes
Speed, kph 0lt v 40 40lt v 50 50lt v 60 60lt v 90 90lt v 110
Frequency 80 15 25 90 30
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Drawing

• Sensible Scales
• Bases correctly aligned
• Plot the Class Boundaries
• Heights correct
• Frequency Density

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Frequency Densities
Speed, kph 0lt v 40 40lt v 50 50lt v 60 60lt v 90 90lt v 110
Frequency 80 15 25 90 30
Frequency Density
Class width
40
10
10
30
20
2.0
1.5
2.5
3.0
1.5
9
Frequency Width x Height
Freq Dens
Frequency 40 x 2.0 80
Speed (kph)
10
How do we treat ages?
Consider this frequency distribution of the ages
of visitors to a department store
Age, Years 15 19 20 24 25 34 35 39 40 - 59
Frequency 60 20 45 70 30
Class width
5
20
5
10
5
Freq Dens
12
4
4.5
14
1.5
15 19 has an UPPER LIMIT of the 20th birthday