the cincinnati, oh–ky–in, msa was originally formed by the united states census bureau in 1950 and consisted of the kentucky counties of campbell and kenton and the ohio county of hamilton. as surrounding counties saw an increase in their population densities and the number of their residents employed within hamilton county, they met census criteria to be added to the msa. the hamilton–middletown, oh msa was also formed in 1950 and consisted solely of butler county, ohio.