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 = 1100  

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

                  Deviation   d = x_i - a

                           x_1 = 900, a = 1100

                                  d = 900 - 1100 = - 200

                    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}

                                                        =   1100 - \frac{6080}{105}

                                              Mean =   1042,1