Mass. Gen. Laws ch. 234A, § 18

At least twelve weeks prior to the commencement of any term of grand or trial juror service, the jury commissioner shall determine, as a matter of discretion, the numbers of jurors to be summoned from each judicial district for grand and trial juror service. In making these discretionary determinations, the jury commissioner shall consider, among other factors, the experience of each court with respect to the numbers of grand and trial jurors who are impanelled or whose services are used during the impanelling process as compared to the numbers of jurors who are summoned.