PhD Failure Rates

According to an article in the Guardian today, over a third of PhD students fail to complete in 7 years. That seems pretty high even by my negative standards. The main reasons they cite are underestimating what a PhD requires, not really understanding what it’s about, over-ambition, lack of funding and supervisory problems. I guess we can all relate to something there. Although the article seems to suggest that for most ‘unfinishers’, the failure to complete the thesis was a bad thing. I wonder how happy the other two-thirds were with their completed doctorate?