The subgroups were designed
to stratify for parity (nulliparous versus parous) and induction
versus spontaneous labor. Hence, the 4 subgroups were
(a) nulliparous women in spontaneous labor, (b) nulliparous
women in induced labor, (c) parous women in spontaneous
labor, and (d) parous women in induced labor.
The patient, attending nurse, and obstetrician were
blinded to group assignment. In some, but not all instances,
the individual collecting data was blinded to group assignment.
After providing consent, patients were assigned to
the next available study number in 1 of the 4 subgroups by
opening the next sequential sealed, opaque envelope. The
anesthesiologist performing the block opened