HIV prevalence in Chicago

A new Chicago Dept of Public Health (CDPH) study finds sobering results about HIV prevalence among men who have sex with men (MSM): of those sampled, 30% of black men, 12% Latino, and 11% of white tested positive.

There's a statistical twist: the reports of the study only give the sample size 570 which is almost irrelevant compared with how they chose these men. Since the detail wasn't given I would guess that they went to stereotypical places, so results would be more accurately reported as being in terms of MSM who go to the stereotypical hangouts where CDPH knew where to find them. I would expect these men to have more sexual partners, more high risk behavior, and so more likelihood of HIV and other STDs. Such high prevalence among any group is of course shocking, but if some omniscient being did a random sample of all MSM, they could pull the partnered, boring, reclusive, etc., and find lower prevalence.

Further, I wonder if their sampling method is differentially selective for higher risk in some groups than others. Gay black men don't have a neighborhood in Chicago comparable to Lakeview, the predominantly white gay neighborhood in Chicago (my former neighborhood), so the more easily sampled may be the very highest risk. In Lakeview they could have found ice cream shops, laundromats, dog supply stores, and yoga classes where upwards of 30% of the clientele are gay; I doubt there's anywhere else in the city with such a high concentration in non-gay-themed gathering spaces. Even if they limited themselves to bars, it might still be that gay bars in Lakeview attract a much broader cross-section of the Lakeview community than African-American gay bars on the South/West side of the city.

Also there is the phenomenon of small numbers: the smaller the group you are drawing from, the more extreme the numbers get. Just two days ago I was estimating chlamydia prevalence among different subgroups of Evangelical Christians and I found a higher prevalence among a specific evangelical denomination of the more frequent church attenders than among evangelical frequent church attenders in general. Are people who have chlamydia attracted to this denomination? Does this denomination discourage condom use more than others? It's possible that was a real effect, and perhaps it will get its own paper, but it's also possible that the higher prevalence was because I was basing my estimate on only 73 individuals, versus 700 for the highly religiously involved.

Youth Pride Services, an agency that serves the African=American gay community in Chicago writes a letter to the editor of the Windy City Times (gay newspaper) saying that they have little visibility. Sadly, notification of this high prevalence may not bring them visibility either.