You parameterize a method when two methods do substantially the same thing, and the only difference is something that could be a parameter. Martin Fowler gives the example of fivePercentRaise and tenPercentRaise methods.
Change:
fivePercentRaise = salary + (salary * 0.05)
and
tenPercentRaise = salary + (salary * 0.10)
to
salary = salary + (salary * increaseFactor)
This is explained much better on page 283 of the Refactoring Book.
See original on c2.com 