"This work estimates the gains in life expectancy, or average years a person lives, at a national level [in Mexico] during the 1950-1980 period by sex and age of the population structure, referring as well to the changes undergone in such gains.  An exhaustive presentation of the Pollard method is made in order to enable us to appreciate the advantages and limitations of the methodology used to quantify the gains in life expectancy. It also shows, graphically, the differences by sex and age from which the gain distributions are derived." (SUMMARY IN ENG)