Answer and explanation:
An effective argumentative essay must provide evidence because evidence is what makes the argument credible and convincing.
The purpose of an argumentative essay is to defend the author's point of view about a certain subject and, ultimately, to convince readers that he is right. To do so, the author must provide readers with evidence that support his opinion. Without evidence, the argument won't have a base to stand on. If, for instance, an author wishes to claim the educational system is worse now than it ever was, for readers to believe him he will have to prove such an argument to be true. By providing statistics, results of studies, and opinions of other authors, he will build a solid base to support his point of view. Chances are he will convince his readers of the truth of his opinion.