2.4 Study selection process and data extraction
To select the studies to be included in the narrative synthesis, one
reviewer first removed the duplicates automatically from the EndNote
library and then revised the records manually. Next, the same reviewer
performed the screening of all the remaining records by checking whether
the publications were related to the reviewed subject, first based on
the title, and second, based on the abstract. Once the screening was
completed, the selected publications were retrieved in full text to
assess their eligibility for final inclusion in the systematic review
and meta-analysis. To account for this, two independent reviewers used a
standardized questionnaire that was based on the eligibility criteria
(Supplementary Table 2 ) and was piloted in 10% of the
publications selected at random. Each time there was a discrepancy
between the reviewers, a third reviewer served as referee until
consensus was reached.
A single reviewer used a standardized questionnaire to extract data from
all the selected publications. Prior to be used, the extraction sheet
format was piloted in 10% of the studies selected at random. The
extracted data included a) the main characteristics of the publication
(first author, country, publication year, and study design), b) the main
characteristics of the assessed population (poultry subgroup, sex, and
age), c) the type of sample reported (birds, products and subproducts,
or environmental), d) the diagnostic technique for NTS, and e) the
reported outcomes (prevalence, serotyping, or antimicrobial resistance).
For the publications that reported both data on the individual and
flock/collective level, we selected for extraction only data at the
individual level to avoid duplication of estimates for a single
publication and because we found less variation in the estimations made
at the individual when compared to the flock/collective level. Besides,
for the publications that assessed more than one type of sample, each
one of them was extracted separately and counted as one independent
study within a single publication. All the extracted data were
registered into an Excel spreadsheet (Microsoft, USA) and a codebook was
created to facilitate its handling.