How do they calculate the percentage of gay people?

With the number of people in the closet, how is it possible to make an accurate prediction? The percentage would have to be a lot higher than the percent that is open about it.How do they calculate the percentage of gay people?
The number of people who identify as gay or lesbian鈥攁nd the proportion of people who have same-sex sexual experiences鈥攁re difficult for researchers to estimate reliably for a variety of reasons. In the modern West, major studies indicate a prevalence of 2% to 13% of the population. A 2006 study suggested that 20% of the population anonymously reported some homosexual feelings, although relatively few participants in the study identified themselves as homosexual.How do they calculate the percentage of gay people?
They are talking about "openly" homosexual.How do they calculate the percentage of gay people?
the census
in some study 80% said almost straight :D

kayaks

textbook