Thank you so much for you quick answer.

I have another question. It looks a bit stupid, but just to clarify. In step 5, the Yt, I guess, it should be in absolute value, isn't it?

For example, if I do the formula without absolute value for the foloowing data: 1300,1450,1600,1850 (mean = 1550).

The deviation without absolute value is 0.
The deviation with absolute value is 700 (the same value if you use AVEDEV formula x 4 in excel).

Thanks again and regards,