American Place Names of Long Ago [not so "long ago" to me!] by Gilbert S. Bahn, a republication of the Index to Cram's Unrivaled Atlas of the World based on the census of 1890 (orig. pub. 1898) Baltimore: Genealogical Publishing Co. 1998 there is the following entry p. 235 "Plainville, Hamilton Co., Ohio. SW [southwest part of county] population [1890] 200. I have found this a very useful book to have and use it extensively. The populations listed may be as small as 8. Many places have the population listed with an "X" indicating the enumerator thought they were too small to have a separate count [about 45% of index]. About 35% of the places had fewer than 100 people.