A Quick Question

So if you watch a TV-sitcom with a laugh track, you'll notice that the jokes get varying laugh times. Some get only a brief chuckle, others a sustained guffaw. But without an actual audience watching the show, how do the producers decide which jokes are "funnier" than their peers? Are the decisions done purely for plot and pacing reasons, or do they actually try and gauge which jokes are their aces and which are the fillers?