Mauro:

You can calculate the errors with Excel's LINEST function. This post gives
an overview of six Excel regression tools.


http://processtrends.com/toc_trend_a...with_Excel.htm

Excel's help menu has a very good discussion of the LINEST function.

Kelly O'Day

"Mauro" wrote in message
...
How can I found the error value of a and b (from the tendence line y=ax+b)
obtained when I plot some data?