Statistical analysis
The distributions of patient, tumor, and treatment characteristics were
compared. Categorical variables were analyzed with the Pearson
chi-square or Fisher exact test, and continuous variables were compared
by one-way analysis of variance. Difference-in-differences analysis for
evaluating a policy change that apply to some but not all members was
used in this series.11,12 In period 1, there was no
specific intervention or policy difference for oral cancer treatment in
target group and control group. In period 2, LNPA was applied for target
group alone. The difference of lymph node yield between different
periods (period 1 and 2), and different departments (target group and
control group) was analyzed. Among difference-in-differences method, the
effects of the target group (target group in period 1), the time period
(period 2), and target group in the post-intervention period (target
group in period 2) was analyzed through liner model for LN yield and
regional recurrence.13 In order to reach the
requirement of strict exogeneity, propensity score matching with
clinical tumor category and node category was
performed.15 A two-side test with a P value of
< 0.05 was set as representing statistical significance.