High School

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Q3: From the following data, find out the Standard Deviation and its coefficient of variation using the Step Deviation Method.

| Age in years | No. of students |
|---|---|
| 50-55 | 25 |
| 45-50 | 30 |
| 40-45 | 40 |
| 35-40 | 45 |
| 30-35 | 80 |
| 25-30 | 110 |
| 20-25 | 170 |

Answer :

To find the Standard Deviation and its coefficient of variation using the Step Deviation Method, follow these steps:

Step 1: Create a Frequency Table

For the given data of ages and number of students:


  • Age 20-25: Frequency (f) = 170

  • Age 25-30: Frequency (f) = 110

  • Age 30-35: Frequency (f) = 80

  • Age 35-40: Frequency (f) = 45

  • Age 40-45: Frequency (f) = 40

  • Age 45-50: Frequency (f) = 30

  • Age 50-55: Frequency (f) = 25


Step 2: Determine the Midpoint (x) for Each Class Interval


  • Midpoint for 20-25: [tex]\frac{20 + 25}{2} = 22.5[/tex]

  • Midpoint for 25-30: [tex]\frac{25 + 30}{2} = 27.5[/tex]

  • Midpoint for 30-35: [tex]\frac{30 + 35}{2} = 32.5[/tex]

  • Midpoint for 35-40: [tex]\frac{35 + 40}{2} = 37.5[/tex]

  • Midpoint for 40-45: [tex]\frac{40 + 45}{2} = 42.5[/tex]

  • Midpoint for 45-50: [tex]\frac{45 + 50}{2} = 47.5[/tex]

  • Midpoint for 50-55: [tex]\frac{50 + 55}{2} = 52.5[/tex]


Step 3: Choose an Assumed Mean (A)

Commonly, the midpoint of the class interval with the highest frequency is chosen as the assumed mean. Here, we choose 22.5.

Step 4: Calculate "d", the deviation from the assumed mean


  • [tex]d = \frac{x - A}{c}[/tex], where [tex]c[/tex] is the class interval (5 in this case).


Calculations for d:


  • d for 22.5: [tex]d = \frac{22.5 - 22.5}{5} = 0[/tex]

  • d for 27.5: [tex]d = \frac{27.5 - 22.5}{5} = 1[/tex]

  • d for 32.5: [tex]d = \frac{32.5 - 22.5}{5} = 2[/tex]

  • d for 37.5: [tex]d = \frac{37.5 - 22.5}{5} = 3[/tex]

  • d for 42.5: [tex]d = \frac{42.5 - 22.5}{5} = 4[/tex]

  • d for 47.5: [tex]d = \frac{47.5 - 22.5}{5} = 5[/tex]

  • d for 52.5: [tex]d = \frac{52.5 - 22.5}{5} = 6[/tex]


Step 5: Find [tex]fd[/tex] and [tex]f(d^2)[/tex]


  • Calculate [tex]fd[/tex] and [tex]f(d^2)[/tex] for each class.





Age Interval
d
Frequency (f)
fd
f(d^2)



20-25
0
170
0
0


25-30
1
110
110
110


30-35
2
80
160
320


35-40
3
45
135
405


40-45
4
40
160
640


45-50
5
30
150
750


50-55
6
25
150
900


Sum of frequencies ([tex]N[/tex]) = 500

[tex]\sum fd = 865[/tex]

[tex]\sum f(d^2) = 3125[/tex]

Step 6: Calculate Mean and Standard Deviation

Mean, [tex]\bar{x} = A + (\frac{\sum fd}{N})\times c[/tex]

[tex]\bar{x} = 22.5 + (\frac{865}{500})\times 5 = 22.5 + 8.65 = 31.15[/tex]

Standard Deviation ([tex]\sigma[/tex]):

[tex]\sigma = c \times \sqrt{\frac{\sum f(d^2) - (\frac{(\sum fd)^2}{N})}{N}}[/tex]

[tex]\sigma = 5 \times \sqrt{\frac{3125 - (\frac{865^2}{500})}{500}}[/tex]

[tex]\sigma = 5 \times \sqrt{\frac{3125 - 1497.62}{500}}[/tex]

[tex]\sigma = 5 \times \sqrt{3.2556} = 5 \times 1.80434 = 9.02[/tex]

Step 7: Coefficient of Variation (CV)

[tex]CV = \frac{\sigma}{\bar{x}} \times 100\%[/tex]

[tex]CV = \frac{9.02}{31.15} \times 100\% = 28.96\%[/tex]

Thus, the Standard Deviation is 9.02 and the Coefficient of Variation is 28.96%.

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