One question I immediately have is that are they controlling for the side that the clerk argues on behalf of? I would expect clerks for "liberal" justices would probably argue on behalf of the liberal side in a SCOTUS case, and likewise for conservative leaning clerks and justices.
In other words: could a liberal side of an argument (say: pro-gay-marriage) hire a former conservative-justice clerk and be more likely to have a conservative justice rule for the liberal argument?