The Obama administration’s directive last week instructing public schools to allow transgender students to use bathrooms and locker rooms of the gender they identify with has set off an intense debate.

Many politicians and parents have raised alarms about privacy and warned of predatory boys sneaking into girls’ bathrooms. But others say that such accommodations are critical protections for a vulnerable population.

For all of the heated debate, a central fact remains elusive: How many students are we talking about?

No one knows for sure. Researchers have not figured out how to obtain consistent, reliable answers from teenagers, much less younger children.