For birth control, there is nothing that is 100% effective. Each type of birth control has its own effectiveness rate. Those choosing a birth control method should look into its effectiveness before making a choice.
In the terms of birth control, effectiveness refers to how often it works to prevent pregnancy.
More About Effectiveness
The effectiveness of human tools and systems is very important. We design things to work as we intend them to. If they do not, we develop them until they are effective.