2.9 Outcomes and prioritization
One important outcome of interest was to gather and summarize data on the frequency of detection from cross-sectional studies, and measures of association (95% confidence intervals) in case-control and case reports studies. For case-control studies the measure of association reported was the odds ratio. In addition to that clinical presentations, type of samples and diagnostic tools used for the detection of porcine astroviruses was reported.
For case-control studies, when the OR was not reported it was calculated if the necessary data were provided. Odds ratios were calculated using a two-by-two frequency table and the 95% confidence intervals were calculated using a formula described by Suzmilas et al., 2010. When there were zero values in the two-by-two table (which make an OR incalculable), 1 was added to each cell when calculating OR for detection of PoAstVs. The statistical software RStudio Version 1.2.1335 was used to create a forest plot using R Package meta (R Core Team R et al., 2013) which summarizes the quantitative findings. A decision was made a priori that if the heterogeneity from the forest plot was significant i.e., P-value of heterogeneity ≤ 0.05, the effects should be summarized using a random effects model (Fletcher et al., 2007). Otherwise, a high P value (P > 0.05) suggests that the heterogeneity was insignificant, and results would be summarized using a fixed effects model.
The subjective reasoning based on the information provided by each paper was used to determine whether each of the nine Bradford Hills criteria was fulfilled. The evidence collected for each Bradford Hill’s criteria presented in results with final judgment as to whether the viewpoint was fulfilled or not.