Data extraction and statistical analysis
An electronic data-collection form was used to extract the following
data from each of the included studies: author, year of publication,
study design, country and period of conduction, number of patients,
demographic characteristics, staging, grading of the tumours, type of
imaging used to stage the neck, criteria used for elective neck
dissection and dissected levels if specified, number of cN0 patients who
underwent elective neck dissection and cases of occult metastases
identified (pN+/cN0). The authors of the selected studies were contacted
in order to gather missing information about individual patient data and
attempt to perform subgroup meta-analysis. Two authors (DB, AV)
independently assessed the quality of the included studies with the
Newcastle-Ottawa Scale(9).
The pooled proportion of occult metastases and corresponding 95%
confidence interval (CI) were calculated according to random-effects
models of DerSimonian and Laird(10), using the logit transformation and
weighting through the inverse variance method. Statistical heterogeneity
among studies was evaluated using the I² and t2statistics. Influence analysis was performed when pooled proportions
were estimated from five or more studies: pooled proportion was
calculated by omitting one study at a time. Publication bias was
assessed through a funnel plot(11).
The results of the meta-analysis were presented graphically using forest
plots, plotting the individual paper, pooled proportions and
corresponding 95% CI. Analyses were conducted using R 3.6, and
statistical significance was claimed for p < 0.05 (two sided).