# Math Assignment Help With Variance Of Grouped Data

## 10.3.3 Variance of grouped data:

Step Deviation Method: When data are grouped into a frequency distribution having class intervals of equal size h, the formula used is;

Example: Calculate the mean and standard deviation for the following distribution:

Marks Number of students

20 – 30

30 – 40

40 – 50

50 – 60

60 – 70

70 – 80

80 – 90

3

6

13

15

14

5

4

Solution: firstly let’s find the mid values of the class intervals and consider a suitable assumed mean, A

Class interval

Frequency

fi

Mid-value

xi

ui = xi – A/10

fi ui

fiui2

20 – 30

30 – 40

40 – 50

50 – 60

60 – 70

70 – 80

80 – 90

3

6

13

15

14

5

4

25

35

45

55

65

75

85

-3

-2

-1

0

1

2

3

-9

-12

-13

0

14

10

12

27

24

13

0

14

20

36

Σ fi = 60

Σfi ui = 2

Σfiui2 = 134

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