I am trying to understand a GWAS paper which reports the effect of a SNP on outcome as a % variation, e.g., 15% variation in xxx levels.

The only information reported are the beta, standard error and p-values. I tried to read the paper but there is no mention of how this is calculated.

The paper in question is Genome-wide association meta-analysis for total serum bilirubin levels (Hum. Mol. Genetics, 2009).

Paragraph in question = These SNPs accounts for approximately 17.5, 18.1 and 16.7% of the variation...

If anyone has any clue, please help. I'll be most appreciative.