Because the functional method permits courts to fashion categories without direction from the statutory text, these courts can create conduct-based distinctions that lack significant differences in degree of risk posed and that stifle uniform application.
The debate over whether to use the formal or functional method to divide a statute and employ the modified categorical approach is an example of the familiar distinction between "rules" and "standards.
Because functional-method courts will divide statutes and employ the modified categorical approach more often than formal-method courts, the functional method presents a more significant Apprendi concern than the formal method.
The functional method treads very closely to exactly such a violation.
There is reason to believe, however, that the formal method will be more likely than the functional method to avoid overinclusiveness, consistent with the values embodied in the rule of lenity.
By consulting the additional documents, functional method courts would, given a sufficient record, categorize defendants' past convictions accordingly.
And because it stays hinged to the categorical approach, the formal method is more likely to avoid overinclusiveness than the functional method.
The functional method, however, uses the modified categorical approach to bypass the statute's vagueness, and if it finds that the defendant's prior conviction is a "crime of violence," it resolves the ACCA's vagueness at the defendant's expense.
It is safe to say, though, that the functional method gives judges more discretion to create categories--principled or arbitrary--divide statutes, and utilize the modified categorical approach.
Here, one might respond that the functional method is harsher where appropriate.
Neither the formal nor functional method may provide states any incentive at ali to rewrite their statutes.