Understanding The Odds
If you work hard at your research then you increase the chance that you get useful outputs for your PhD.
If you write the best thesis you can then it’s more likely it will be read well by someone else (like an examiner!).
If you prepare for your viva then odds are you’ll be better off than if you had done nothing in particular.
And if you do all of these, as most PhD candidates do, then you’re very likely to succeed.