What qualifies as sufficient evidence to prosecute?

As far as English law is concerned, the way it goes is that the police collect evidence, if they think this is reasonably convincing they pass it all to the Crown Prosecution Service, and it is their job to decide whether to bring charges. The criterion is that the CPS should be reasonably sure of a conviction, or it's a waste of time and money bringing charges. "Reasonably sure" is the answer.

It works on the whole - 80% of defendants in English magistrates' courts and Crown Courts plead guilty (and generally this is because they obviously are - "what evidence do I have in my defence?" If it's obvious that the police evidence is so overwhelming that you're obviously guilty, you're an idiot to do anything else, and doing that reduces the sentence). Another 10% get thrown out when more evidence comes to light, and only the remaining 10% go to trial. And then the bench of magistrates or the jury listen to the evidence and have to be convinced "beyond reasonable doubt" for a conviction. I've done jury service and the judge told us that - "you must have no reasonable doubt if you're going to convict". That's a tougher standard than for bringing the case to start with. So just under half of cases going to trial result in a verdict of not guilty.

As Bruce says, obviously American but the UK is the same, fingerprints in your own house prove nothing, but if they're found somewhere you shouldn't have been, there's a question - how did they get there?

It's quite a live issue at the moment, given all the celebrities that have been accused of sex offences since the whole thing about Jimmy Savile blew up. These really are difficult cases because it's all "my word against yours" given that so much of this supposedly happened many years ago. The CPS has brought charges, and in every case so far it has been proved wrong - or at least the jury thought it was wrong. The only one who is in prison for any of this is Stuart Hall, who admitted it, pleaded guilty, and therefore it never went to trial. So is the CPS being too trigger-happy in bringing prosecutions that will fail?

Perhaps I can give some insight because when I was called for jury service 14 years ago, I got picked on the first day for a case of a man who sexually abused all his own daughters. How the police found out about it was that his 10 year old daughter had a friend from school to stay for a sleepover, he did wrong things with her, she went home and told Mum, and quite reasonably she went totally ballistic and told the police. So we had four girls all telling us on the jury about this. We listened to them for a week. The police did it really well - the 10 year old daughter never came into court at all and the court played us the video of her talking to a friendly policewoman in a playroom, so she could feel safe and open up. It was easy for us to convict with 4 girls all saying the same thing, and it wasn't so long ago, but what about Michael Le Vell, where only one girl accused him of anything? I can well imagine the jury couldn't be so sure. And if they aren't sure, they mustn't convict.

Probable cause is what qualifies evidence. If the evidence suggests you were likely the one that committed the crime, then you can be charged. The evidence needed for a criminal conviction is much higher than what is needed for arrest. A conviction requires proof beyond a reasonable doubt.

Fingerprints are of no value at all unless they are where they do not belong. For example, if your roommate accuses you of stealing money, your fingerprints would be expected to be all over the place because you share the house with him. If you are accused of burglarizing a house that you should have never been in, fingerprints will be very valuable.

your playing with words here

if evidence is sufficient or not is down to the prosecutions opinion what someone might think is strong can be made to look weak by the defence

Prosecutors, like District Attorneys, decide when there is enough evidence to warrant proceeding to court.

For any criminal prosecution to take place one must have "evidence beyond reasonable doubt".

Ack a DA.