“Matthew Shugart”:http://fruitsandvotes.com/?p=4231 has a good post.
bq. Building on a discussion in one of the Australia post-election threads, I checked the data for Presidents, Parties, and Prime Ministers to see if PMs in “Westminster” systems were any less frequently dismissed due to “intra-party conflict” than PMs in parliamentary systems overall. To make a long story short, they are (except, apparently, in Australia). … There are only nine of 78 PMs in Westminster systems who leave for reasons that might be termed intra-party conflict. That’s 11.5%, which I have to agree is a good deal less than 3 of 10! What about in the non-Westminster cases? We should also remove the “voluntary” departures from this subset. When we do, we are left with about 24% leaving due to intra-party conflict in the entire parliamentary data. In the non-Westminster subset, it’s 28%. So there you have it. Roughly one eighth of Westminster PMs leave due to intra-party conflict, compared to well over a quarter of non-Westminster PMs. That seems pretty significant.