Mass. Gen. Laws ch. 234A § 12
On or before the first day of July of each year, the office of jury commissioner shall determine the number of prospective jurors to be drawn from each city and town. This number shall be as nearly as possible equal to the ratio of the population of the city or town to the entire population of the judicial district in which the particular city or town is situated, multiplied by the total number of prospective jurors required for the judicial district. The total number of prospective jurors required for each judicial district shall be determined by the jury commissioner as a matter of discretion. The population of the cities, towns, and judicial districts required under this section shall be determined from the numbered resident lists.
Mass. Gen. Laws ch. 234A, § 12