I am 18 months into my PhD. I have some useful data, enough for maybe half a chapter or a maybe a full chapter at a push. I have more experiments lined up to collect lots more data. I am constantly worrying however that what i'm doing may not be enough come the end of my 4 years. My supervisors tell me that I am on the right track, but with the proviso that what I achieve in the end will depend on the results I obtain from my experiments. If you make it to the viva is there a good chance you can still fail even with your supervisors insisting you're on the right track? Thank you

If you make it to the viva you could still fail but here's how I'd put the likelihoods:

1. Pass with no corrections: Very rare these days (I'd say 1% of viva candidates)
2. Pass with corrections: Common outcome (I'd say 40% of students get this)
3. Revise and Resubmit: Also common (I'd say also 40% of students get this)
4. Get an Masters in place: Actually more common than people think (around 18%)
5. Fail outright: Very unlikely (around 1% as well)

That's just based on my uni, other unis, online bloggers, etc… What your supervisor tells you is important so if he thinks you're on the right track it's likely you are but it's no guarantee that the bad outcomes won't occur.

Having said that at 18 months in it's still too early to make impressions. Stick to your work and I'm sure you won't get any of the bad outcomes.