Document PHSKC / BRFSS-based MSM prevalence as third reference point

[smjenness]

Context

Public Health Seattle-King County has historically used a BRFSS-based estimate of identity-based MSM prevalence (5.7% pre-2014, 6.5% as of 2021). This is an independent third data point on the question, methodologically distinct from both Grey (ACS households + NHANES urbanicity weights + Purcell anchor) and Islek (Census 2020 households + NHANES direct). The PHSKC value aligns much more closely with Grey/Purcell than with Islek.

This issue documents the PHSKC/BRFSS approach and assesses whether it can be replicated for our other settings.

Investigation tasks

• Document PHSKC's stated methodology from the 2022 HIV/AIDS Epidemiology Annual Report: BRFSS rolling 3-year average, identity-based proxy.
• Compare PHSKC's identity-based 6.5% to Grey's 6.1% and Islek's 2.4% for the Seattle CBSA. Note this alignment is one of the strongest external sanity checks available.
• Investigate BRFSS geographic detail: state-level data should be available; some MSAs may have CBSA-level estimates via SMART BRFSS.
• Attempt to replicate the PHSKC approach for Georgia / Atlanta MSA using BRFSS state SMART files. Document feasibility.
• Discuss the identity-vs-behavior gap: BRFSS asks about gay/bisexual identity, NHANES about behavior. What is the typical ratio?

Priority

Medium. Provides a triangulation point for the Grey-vs-Islek decision.

Source

Plan: inst/popsize/msm_population_size_discussion.md, §Q4.
Reference: inst/popsize/2022-hiv-aids-epidemiology-annual-report.pdf (PDF page 6).