Subject: Why g differs from experimental g, even when adjusting for Inertia?

Using an Atwood machine, with pulleys, I determined the experimental value of 
g, taking into consideration inertia and pulley mass?  Why is the percent error 
so large - 33%?  Derived friction force from a line graph - potential for error 
here.  Also, errors in measuring time possible.  But if there were no human 
errors, would the accepted value of g equal the derived value if equation 
controlled for inertia and pulley mass?

Re: Why g differs from experimental g, even when adjusting for Inertia?