Both assumed mean and step deviation methds are way to find the mean of given data.

1. Assumed mean method:

Step1. Tabulate the continuous class intervals and frequency along with class marks.

Here, Class Mark = (Upper limit + lower limit)/2

Step2. Now, assume any of the class mark to be as mean.

Step3. Take the difference of the assumed mean and class marks

i.e. x_i - a = d_i

Ex: If the class marks are 20, 30, 40, 50, 60, ..... and if a  = 40

then d_i is -20, -10, 0, 10, 20, .....

Step4. Multiply f_i and d_i

Step5. d = sum of f_i * d_i /sum of f_i

Step6. Mean = a + d

2. Step deviation method:

Step1. Tabulate the continuous class intervals and frequency along with class marks.

Here, Class Mark = (Upper limit + lower limit)/2

Step2. Now, assume any of the class mark to be as mean.

Step3. Take the difference of the assumed mean and class marks

i.e. x_i - a = d_i

Ex: If the class marks are 20, 30, 40, 50, 60, ..... and if a  = 40

then d_i is -20, -10, 0, 10, 20, .....

Step4. Calculate u_i = d_i /h             {h = class size}

Step5. Multiply f_i and u_i

Step6. Calculate U = sum of  f_i * d_i /sum of f_i      {U = u bar}

Step7. X = a + U*h                                         {X = x bar}