The variety in the tasks encompassed by sex work lead to a large range in both severity and nature of risks that sex workers face in their occupations.
Maintaining privacy and confidentiality in research is also difficult because many sex workers may face prosecution and other consequences if their identities are revealed.
Exner, an American psychologist, worked with his fellow colleagues to create five distinct classes for categorizing sex workers.
One scholarly article details the classes as follows: "specifically, the authors articulated Class I, or the upper class of the profession, consisting of call girls; Class II was referred to as the middle class, consisting of 'in-house girls' who typically work in an establishment on a commission basis; Class III, the lower middle class, were 'streetwalkers' whose fees and place of work fluctuate considerably; Class IV sex workers have been known as 'commuter housewives', and they are typically involved in sex work to supplement family income; and Class V consists of 'streetwalker addicts', or 'drugs-for-sex streetwalkers' who are considered the lower class of the profession".
Many studies struggle to gain demographic information about the prevalence of sex work, as many countries or cities have laws prohibiting prostitution or other sex work.
In addition, sex trafficking, or forced sex work, is also difficult to quantify due to its underground and covert nature.