Tests how often the SNP-exposure and SNP-outcome signs are concordant. This is to avoid the problem of averaging over all SNPs, which can suffer bias due to outliers with strong effects; and to avoid excluding SNPs which is implicit in median and mode based estimators. The effect estimate here is not to be interpreted as the effect size - it is the proportion of SNP-exposure and SNP-outcome effects that have concordant signs. e.g. +1 means all have the same sign, -1 means all have opposite signs, and 0 means that there is an equal number of concordant and discordant signs. Restricted to only work if there are 6 or more valid SNPs.