THE SHORT ANSWER
Two things. Breach of duty means the care fell below the standard of a responsible body of professionals in that field. Causation means that substandard care caused your injury or made it worse. Both are decided on the balance of probabilities, which means more likely than not, and both rest on independent expert evidence.
People often assume a claim turns on one question: was the care bad? In fact it turns on two, and claims are lost on the second one just as often as the first.
Understanding both explains why some cases that sound strong do not succeed, and why others that sound minor do.
Breach of duty: was the care below standard?
Breach of duty means the care fell below the standard of a responsible body of professionals in that field. This test comes from a case called Bolam.
The courts later added an important safeguard, in a case called Bolitho. It is not enough for a doctor to find a colleague who would have done the same thing. That opinion has to stand up to logical scrutiny. A practice does not become acceptable just because someone else follows it.
In practice, this means both sides instruct an independent expert in the relevant specialty, and the argument is between two informed professionals views rather than between you and the hospital.
Causation: what difference did it make?
Causations asks whether the substandard care actually caused your injury, or made it worse. It is a separate question and it is often the harder one.
• A cancer should have been spotted on a scan in March, but was not diagnosed until September.
• The delay may well be a breach of duty. Causation asks something different: what difference did those six months make to your treatment and to your outlook?
• If the treatment and the prognosis would have been the same either way, there may be no claim despite the delay. If the cancer progressed in that time and the treatment became more invasive or less likely to succeed, that is the harm the claim is about.
This is why a claim needs a clear picture of what should have happened, not just what did. Establishing that takes medical records and expert evidence, which is why the early stages of a claim take time.
What does “balance of probabilities” mean?
Both breach and causation are decided on the balance of probabilities. That simply means more likely than not.
It is a lower bar than the criminal standard, which requires a jury to be sure. You do not have to prove what happened beyond doubt. You have to show that your account is more likely than the alternative.
In practice, a case that rests on a fifty-fifty split will not succeed, because it does not tip past more likely than not.
Who decides: the role of independent medical experts
The evidence on both questions comes from independent medical experts. Not from us, and not from the hospital.
An expert is instructed because they practice in the relevant specialty and can say what a reasonably competent professional would have done. Their duty is to the court, not to whoever instructed them, and they are required to say when the evidence does not support the case.
That is worth knowing if you are worried the process might be one-sided. An expert who reviews your records and concludes the care was acceptable will say so, and that is usually the point at which a claim stops.
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Wondering whether your case gets past both tests?
It is not something you can work out from the outside, and you are not expected to. Tell us what happened and we will explain what the records would need to show.
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The Bolam test asks whether the care met the standard of a responsible body of professionals in that field. Following a later case called Bolitho, that professional opinion also has to stand up to logical scrutiny.
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Causation means showing that the substandard care caused your injury or made it worse. It is separate from proving the care was poor, and a claim needs both.
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If your treatment and your outlook would have been the same either way, a delay may not support a claim even where the care fell below standard. This is a causation question and it needs expert evidence to answer properly.