Stata software package version 13

Data were entered into Microsoft Excel 2007 sheets, double checked against written data collection forms and then transferred to Stata® Software package, Version 13 (Stata Corporation, College Station, Texas 77,845 USA, stata@stata.com) for analysis. Descriptive statistics were generated.

Data was entered into excel sheets (Microsoft Office, 2007) and validated by checking all entries against all of the original data collection forms. Then data was exported to Stata^® Software package, Version 13 (Stata Corporation, College Station, Texas 77845 USA, stata@stata.com) for analysis. Spearman’s rank correlation coefficient was calculated to assess the relationship between pairs of continuous groups of variables which were not normally distributed. The Wilcoxon rank sum test was used to establish if there were differences in the number of lesions in animals between two groups or predilection sites. Mid p-exact tests were conducted using the rate2by2.test function as implemented in the epitools package in R-software to establish the significance of differences between proportions. Only p-values of less than 0.05 were considered statistically significant.

The main findings were reported in a tabular synthesis, separately for each SNP, and the qualitative synthesis reported possible associations of each SNP with the SPC risk. Meta-analysis was performed considering different study designs, and the SNPs studied reported in each study. When at least two studies on the same SNP were available and evaluated the same genotype model, the data were pooled in a random-effect meta-analyses [20 ]. Effect size were expressed as hazard ratios (HR) or odds ratios (OR) with the corresponding 95% confidence intervals (CI), as appropriate. We stratified the analyses according to the site of SPC. The heterogeneity between studies was assessed using the χ2-based Q-statistics and the I² statistics [21 ]. The heterogeneity was considered low if the I² value was < 25%. P-values of less than 0.05 were considered statistically significant. To assess the presence of publication bias (where appropriate), we conducted Egger’s asymmetry test (level of significance p < 0.05) for the SNPs with at least three pooled studies [22 (link)]. Statistical analyses were performed using the Stata software package version 13 (StataCorp. College Station. Texas).