You can’t flip a coin to determine viva success.
The stories we tell about vivas pivot on knowing if someone passed or failed – but these things are not equally likely. Around one in one thousand PhD candidates don’t pass.
There are many, many reasons why candidates pass – the process, the structure of the PhD, the skillset and knowledge base and experience that a candidate must typically have.
Doubting your future success is a human response; knowing a little of what to expect in the viva can be the first step to putting doubts to one side, so you can focus on being ready to succeed.
Passing the viva is not due to simple luck.