Meta-regression analyses were used to explore the source of heterogeneity of response, using the individual SMD for each study comparison as the outcome and the associated SE as the measure of variance. Meta-regression is also used to explore sources of heterogeneity even when the initial overall test for heterogeneity is not significant (Higgins and Thompson, 2002). The duration, inclusion level, strain and presentation form were tested as meta-regression variables for the broiler performance data set. Variables were considered to be a source of heterogeneity if they produced a P-value < 0.05.
Results and Discussion
Studies Identified and Information Extracted
The literature review yielded 21 independent studies reporting the effects of dietary ginger supplementation on production responses of chickens in Pubmed, AGORA, Science Direct, Google scholar and CAB (Table 1). All the studies included in the dataset were published in peer-reviewed scientific journals. There were 6 duplicates studies removed and 12 studies excluded based on relevance of title and type of articles (i.e., abstracts, theses, and research reports were excluded). A total of 21 studies met the inclusion criteria for one or more of the meta-analyses conducted and were used in the meta-analysis analysis. Most of the primary studies included in the meta-analysis contained a multiple comparison with a single control such as different levels of ginger, whereas other studies contained a summary of a single comparison between control and ginger. Each variable was not always reported in each study.