Measuring the relationship between party control and economic growth

http://arts.bev.net/roperldavid/politics/scores.htm

I had in mind to improve my stats chops and measure the impact that Dem/GOP control of the executive/legislative branches has on economic growth. The linked site beat me to it, however there are some deficiencies there, including a lack of tests of statistical significance and a relatively arbitrary quantification of growth, not to mention a totally ad hoc measure of control. I'd love to follow this up by investigating a full model using GDP as growth indicator and testing for significance.