Evaluate value added of a company i  have the value  added  of  a  company for 2 years  2016  and  2017  so  to  compare these  values and to see its evolutioon i divided the value of 2017 by  the one of  2016

but  some friends told me that the comparison i am doing is false because  values should  be calculated using  same  price  base

so  what should i do to have a correct  evaluation  ?

thanks a lot  in advance for  any help
What you are missing is the concept of GDP deflator .
Which is there while studying National Income Accounting.

When you are comparing change in Valve Added (GDP) of 2016 and 2017 , the change could have been mainly due to two reasons :-


Increase in production
or Change in prices


While comparing Value Added by the company in two years , second reason i.e change in prices would give us false result as while comparing GDP we want to see how much the production has increased/decreased between the years , but the change in prices might show an exaggerated value of GDP if we do not take into account the change in prices .

For getting the correct answer what we do is that we take one year as base year(generally the first year given to us i.e 2016 in your case) and calculate the GDP on the basis of the prices prevalent on that year , so that we get a true picture of real change in GDP .

$$
\text{GDP Deflator} = \frac{\text {Nominal  GDP}}{\text{Real GDP}} \cdot {100}
$$

Where Nominal GDP is the GDP of 2017 at current year i.e 2017 prices 
And Real GDP is the GDP of 2017 calculated at base year prices i.e 2016

Also remember the formulae $\text{GDP = Units produced * Price per unit} $

Do refer to the following hyperlink


GDP deflator


Hope you find my answer useful. :)