Step 1: Know about the deviation or assumed mean method

          The assumed mean method (or deviation) 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_id_i}{\Sigma f_i}

                      Where, d_i = x_i - a

                            a - Assumed mean

Step 2: Choose one among the observations as the assumed mean, and denote it by 'a'

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

                        Assumed mean a = 75.5

Step 3: Find the deviation of ‘a’ from each of the observations ( x_i 's)

                    Deviation d = x_i - a

                         x_1 = 66.5, a = 75.5

                          d = 66.5 - 75.5 = - 9

Calculate the deviation for every observation (See the table)

Step 4: Find the product of d_i(deviations) with the corresponding frequencies f_i, and take the sum of all the f_id_i's.

                Calculations are shown in table given below


Step 5:  Find the mean of the deviations

                Mean of the deviations = a + \frac{\Sigma f_id_i}{\Sigma f_i}

                                                      =   75.5 + \frac{12}{30}

                                          Mean = 75.5 + 0.4

                                            Mean = 75.9