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.

## Usage

`mr_sign(b_exp, b_out, se_exp = NULL, se_out = NULL, parameters = NULL)`

## Arguments

- b_exp
Vector of genetic effects on exposure

- b_out
Vector of genetic effects on outcome

- se_exp
Not required

- se_out
Not required

- parameters
Not required

## Value

List with the following elements:

- b
Concordance (see description)

- se
NA

- pval
p-value

- nsnp
Number of SNPs (excludes NAs and effect estimates that are 0)