S2A

Chapter 7

More about Statistical Graphs

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Terminologies about Classes

19.5 24.5

Lower class

boundary

Upper class

boundary

20 2415 19 25 2920 24

Upper and lower class limits

Class width 24.5 19.5 5

Class mark

22

22420

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Frequency Distribution TableWaiting Time of 100 Passengers at a Bus Stop

Waiting time (min)

Class boundaries (min)

Class mark (min) Frequency

5 - 9 4.5 - 9.5 7 16

10 - 14 9.5 - 14.5 12 18

15 - 19 14.5 - 19.5 17 20

20 - 24 19.5 - 24.5 22 17

25 - 29 24.5 - 29.5 27 14

30 - 34 29.5 - 34.5 32 11

35 - 39 34.5 - 39.5 37 4

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Frequency Polygons

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Cumulative Frequency Table

Waiting time less than (min) Cumulative frequency

4.5 0

9.5 16

14.5 34

19.5 54

24.5 71

29.5 85

34.5 96

39.5 100

2009 Chung Tai Educational Press. All rights reserved.©

Waiting Time of 100 Passengers at a Bus Stop

0

20

40

60

80

100

120

4.5 9.5 14.5 19.5 24.5 29.5 34.5 39.5

Waiting time (min)

Cum

ulat

ive

freq

uenc

yCumulative Frequency Polygon

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Percentiles

• Percentile represents a given percentage of the data less than a given value.

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Change the cumulative frequency to percentage.

For example: 74% of students got the examination scores below 66.4.

Percentiles

100%

90%

80%

70%

60%

50%

40%

30%

20%

10%

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• The score of 66.4 is called the 74th percentile and is denoted by P74. It means 74% of S2 students have scores less than 66.4.

• If there are 200 students in total, there are 148 (74% 200) students have scores less than 66.4.

Percentiles

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Quartiles• The percentiles P25, P50 and P75 divide the

number of data into 4 equal parts, so they are called quartiles.

• P25 is also called the lower quartile (denoted by

Q1).

• P50 is also called the median (denoted by Q2).

• P75 is also called the upper quartile (denoted by

Q3).

The End