We made the assumption that the odds ratios from case - control studies approximate the hazard ratios from cohort studies . To obtain pooled relative risk estimates we weighted the log of the odds ratios or hazard ratios by the inverse of their variance . We ran random effects models , which include assumptions on potential variability across studies . We used the DerSimonian and Laird Q test for heterogeneity . Since this test statistic has low power with studies of small sample size and excessive power if there are many studies ( especially if they are large ) , we also calculated the I 2 statistic for each analysis . 8 This statistic describes the percentage of total variation across studies that is due to heterogeneity rather than chance ( 25 % low heterogeneity , 50 % medium , 75 % high ) . We used Galbraith plots 9 to visually examine the impact of individual studies on the overall homogeneity test statistic , and we used meta - regression to evaluate the amount of heterogeneity of study type , sex , and migraine aura status on some of the cardiovascular events . We evaluated potential publication bias by visually examining for possible skewness in funnel plots 10 and statistically with the methods described by Begg and Mazumdar 10 and Egger . 11 Egger 's method uses a weighted regression approach to investigate the association between outcome effects ( log odds ratio or log hazard ratio ) and its standard error in each study . Analyses were carried out with Stata 8.2 . Figure 1 summarises the selection of studies . Of 5746 potentially eligible articles identified through the electronic search , 5714 were excluded after an evaluation of the title and abstract . A further seven were excluded : two used the same cohorts as other studies but had shorter follow - up , 12 13 one did not present confidence intervals for the relative risks , 14 one was cross sectional and did not present overall estimates for the follow - up data , 15 one was cross sectional and the temporal association between migraine and cardiovascular events was not clear , 16 one used only descriptive statistics , 17 and one was a case - control study from a database that also contained a cohort analysis . 18 Overall , 25 studies were suitable for inclusion in the analysis ( table 1 ) .