It is more effective to use more than one method.
For example, the failure rate of the Pill in practice is thought to be
around 9% per year. If you start with 100 women, then 9 would likely get
pregnant.
The failure rate of a condom and spermicide is thought to be around 13%,
ane similarly starting with 100 women, 13 then would likely get pregnant.
Now if you were to use both methods, things are much better, for example,
of the the 9 that might have gotten pregnant on the pill alone, theoretically
only 13%--or 1 of them--would likely get pregnant using both methods.