I'm not sure how Veritas does it, but 23andMe only takes a few of the 600K SNPs available to analyze. PBS had a good bit on it, you can find it on YouTube. They release your raw data, but make no guarantee of accuracy, which is crazy considering some people take drastic, irreversible medical action (double mastectomy, hysterectomy for the BRCA mutation) based on the results. I'm sure they disclaim that, but crazy nonetheless.