1. Arrange the data in a table by quarters and years.
2. Calculate the average of quarters for years 1 and 5. Substract 5 from and divide the result by 16. Call it net upward movement over 16 quarters, not 20. And get the average.
3. calculate grand mean for every quarter
4. Elliminate num by adding 1.5 num to Q1, 0.5 to Q2, 0.5 to Q3, 1.5 to Q5  get the adjusted average.
5. Calculate the mean of quartely grand means.
6. Divide the quartely average by the whole average and get the seasonal pattern
7. Divide original figgures by the seasonal pattern to get the seasonally adjusted data.
year \ Quarte 
Q1 
Q2 
Q3 
Q4 
Average 
1 
100 
70 
60 
90 
80 
2 
120 
85 
70 
110 

3 
150 
105 
85 
130 

4 
170 
125 
100 
155 

5 
190 
150 
120 
180 
160 
average 
146 
107 
87 
133 
Grand118.3 
Netward 06ve0ent = 80, Average num=5
Adjust num 
1.5x5 
0.5x5 
0.5 
1.5 

Ratio to gran 
1.29 
0.92 
0.71 
1.06 
1.0(seasonal 
Estimation 
Q1 
Q2 
Q3 
Q4 

YEAR 1 
77 
75.6 
83.9 
84.8 

2 
92.4 
91.8 
97.9 
103.7 
It is essential to have the same number of quarters in every year. (ie not only first)
Awkward factors like easter
Changes in practice  holidays arrangement with Bank holidays
Do not use for more than 5 years