Invited review: Use of meta-analysis in animal health and reproduction: Methods and applications

Sample size per group for an intervention targeted at a 50% reduction in incidence, based on estimated incidence of disease in the control group. For example, milk fever, dystocia and abomasal disorders (white bar), ketosis, retained placenta (gray bar), and metritis, mastitis and lameness (black bar). From Lean et al. (2008).

Risk ratios (RR) [and their 95% confidence interval (CI) and weights for individual trials] determined from the results of 42 trials comparing risk of pregnancy (PR) conception at 1st and 2nd insemination and overall risk of pregnancy in dairy cows injected with GnRH or analog at the time of insemination in cows at first and second insemination and repeat breeder cows (data obtained from Morgan and Lean, 1993). Box sizes are proportional to the inverse variance of the estimates. Summary estimates of treatment effects are shown using: 1) a fixed effects approach (Mantel-Haenszel = M-H subtotal), 2) a random effects approach (DerSimonian and Laird = D+L subtotal), and 3) the estimated predicted interval of a future trial; this prediction can provide further evidence of the effectiveness of treatment in future studies. A full list of the studies referenced here is available online (http://jds.fass.org/content/vol92/issue8/).

Study designs that are used in animal and veterinary science, indicating strength of evidence of causality and whether the study design is of greater internal or external validity (modified from Dohoo et al., 2003).

Galbraith plot for assessing heterogeneity in Ovsynch and prostaglandin F_2α comparison data in lactating dairy cows (developed using the data from Rabiee et al., 2005).

Funnel plot of blood glucose data in lactating dairy cows supplemented with monensin. x-axis is the standard error (SE) of standardized mean difference (SMD), and y-axis is the standardized mean difference (SMD) (data from Duffield et al., 2008a).

Funnel plot of blood β-hydroxybutyrate data in lactating dairy cows supplemented with monensin. x-axis is the standard error (SE) of standardized mean difference (SMD), and y-axis is the standardized mean difference (SMD) (data from Duffield et al., 2008a).

Contour-enhanced funnel plot of pregnancy rate in cycling and repeat breeder dairy cows injected with GnRH at the time of insemination (data from Morgan and Lean, 1993).

Influence analysis for monensin effect (SMD) on 3-hydroxybutyrate (Duffield et al., 2008a). A full list of the studies referenced here is available online (http://jds.fass.org/content/vol92/issue8/).

Unpublished figure illustrating the influence of a single study on the impact of monensin on the relative risk (RR) of dystocia (Duffield et al., 2008c). The influential study is Melendez et al. (2006). The overall RR of dystocia with the study included is 1.39, whereas it is 1.07 with the study removed. A full list of the studies referenced here is available online (http://jds.fass.org/content/vol92/issue8/).