Not sure if there’s a better approach since this was originally posted, but comparing apples to apples here, third party content (probably not too surprising) has really ballooned since @igrigorik first ran the query in Nov of 2013.
The first party counts have stayed mostly the same, very slight variation. But 3rd party is definitely a different story. That’s 52.8% 3rd-party components at the median, now.