I found a couple of reports on the Massachusetts Dept. of Corrections website that might be helpful -- one concerns commitments of women at the state level (http://www.mass.gov/Eeops/docs/doc/research_reports/237_commits.pdf), the other at the county level (http://www.mass.gov/Eeops/docs/doc/research_reports/219_commits.pdf)-- between 1970-1980, broken down by year. The state-level report mentions alternative sentencing practices, but in terms of data, the breakdown is between three types of sentences: county, reformatory, and prison. From what I can tell from my (admittedly cursory) reading, the county report mentions alternative sentencing but does not provide data about how many women received alternative sentences.
For additional data or reports published before 1976, the DOC website suggests that researchers contact the Research and Planning Division via email: research[at]doc.state.ma.us. The Division's phone number is (978) 405-6677.
How and where would I find the number of women imprisoned in Massachusetts in 1971, 1972, and 1973? (I understand that the Massachusetts Department of Corrections had planned to phase out its female prisons by April 1973. If this did indeed happen, how would I find out the number of women sentenced to prison alternatives that year?)