Inclusion Criteria and Data Extraction
The authors employed several selection criteria to ensure that highest standard was met. Studies that reported the presentation form of ginger (powder, extract, oil), randomization procedure used, at least one of the production outcomes and a measure of variance (SE, SD, 95%CI) for each effect size were selected.
In carrying out meta-analysis, standard deviation (SD) is a vital input variable and a condition where the standard error (SE) of the production outcome of choice was provided instead of SD, the SD is estimated the formula (SD = SE×?n) of Higgins and Deeks (2011). Where “n” is the number of replication. In an event where neither SE nor SD was reported, and the authors were not able to contact the corresponding authors, SD was reassigned as 10% of the mean of the production outcome of interest (Gattinger et al., 2012; Zhao et al., 2017). The effect of the estimation method for the SD was assessed via sensitivity analyses.
The set inclusion criteria for the meta-analysis were met by 21 studies out of the 30 studies. Our inability to get additional information needed to execute meta-analysis from the corresponding authors and absence of uniformity in analytics was the basis for the rejection of 9 candidate primary studies. Finally, a dataset was built from twenty-one peer reviewed journal papers and data extracted from these primary studies included performance data (weight gain, feed intakes, feed conversion ratio), organ weight (liver, heart, gizzard and abdominal fat), total lipids (triglycerides, cholesterol, high density lipoprotein, low density lipoprotein) and semen quality (semen volume, sperm concentration, sperm motility, live spermatozoa and abnormal spermatozoa). The extracted data also included first surname of the first author, year of publication, duration of supplementation, chicken type, country where the study was executed, number of birds used in control and treatment groups and the measures of variance of responses (SE, SD or p-value).