 Krishna
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Step 1: Know about the step - deviation method

The step-deviation method  method are just simplified form of

the direct method.

Let x_1, x_2, x_3............, x_n be observations with respective

frequencies f_1, f_2,............f_n

Mean = a + \frac{\Sigma f_iu_i}{\Sigma f_i}*h

Where, u_i = \frac{d_i}{h}

d_i = x_i - a

a - Assumed mean

h - is the class size.

Step 2: Choose one among the observations as the assumed mean, and Find the deviation of ‘a’ from each of the observations

NOTE: It is taken somewhere in the middle of all the values of observations

Assumed mean a = 70

Deviation d = x_i - a

x_1= 50 , a = 70

d = 50 - 70 = - 20

Step 3: Calculate the deviation for every observation (See the table)Divide the deviation by the class size (h) to calculate u_i

u_i = \frac{d_i}{h}

Class size (h): Generally size of the class is taken as h but it need not be

size of the class always.

Let us still apply the step deviation method with a = 70 and h = 55 - 45 = 10.

Then, we obtain the data as given in the table. Step 4: Calculate the mean using the step deviation formula

Step deviation mean = a + \frac{\Sigma f_iu_i}{\Sigma f_i}*h

= 70 + \frac{-2}{35}*10

Mean = 70 - 0.5714

Hence, Mean = 69.42