A possible effect modification was observed
in two subgroups: subgroup analyses suggested
that very low pressure was superior to low or
high pressure in patients with a tibial fracture
but inferior in patients with other fractures
(P = 0.05 for interaction) and that saline solution
was superior to soap when antibiotics were given
for less than 4 days after surgery, but saline and
soap were similar when antibiotics were given
for 4 or more days (P = 0.03 for interaction). Because
many subgroup analyses were performed,
the positive results in these two subgroups have
relatively low credibility.30