A "confession", to be of any use, should reveal facts about the crime that would not be known to someone who didn't actually do it. A confession that fails to do that is of little value. However, false confessions are frequently given enormous value by police and jurors.
Truism.
Why bother to make a statement which is intended to be self-evident? "The sun will appear on the eastern horizon in the morning." Big deal.
To be accurate, though, your statement should reflect that a confession should reveal facts which someone who didn't actually do it had not had an opportunity to learn. This is a more stringent condition than yours.
It is also wrong. This is a condition which makes a confession less assailable, but it is not a necessary one. If all the details of a particular crime are generally known it cannot be satisfied. The willingness of a particular individual to admit to the act itself cannot always be evaluated solely on that premise. Confirmed, maybe, but not eliminated.
I am not aware that Knox's statement revealed facts about the crime that were previously unknown, and the fact that police failed to record their questioning of her makes it impossible to know what she told them and what they told her anyway.
I thought that Knox's statement, or "confession" (or whatever we choose to call it

) placed her at the scene of the crime during its execution. This is in direct contradiction to her earlier statements. True or false, voluntary or "induced" that would make any discussion of facts "that would not be known to someone who didn't actually do it." utterly irrelevant.
I don't understand the point you're trying to make about statistics.
This is very evident. You appear to be in good company here in that regard, or a lot of it, at any rate. Whether this failure to comprehend is willful or not I do not know.
I cannot reduce the concept to terms any more simple than I have in this and earlier conversations on the same topic. An anecdote is not evidence. "The plural of 'anecdote' is not 'evidence'." "Anecdote" is not a pejorative. It is a description of a datum, one instance to include in a data set. Of itself or even in aggregate it provides no evidence for conclusion, only for hypothesis ... at best. You and others here incessantly bring forward examples of false confession with the sinister insinuation that it is commonplace or perhaps even more common than not. For this to constitute anything
beyond mere insinuation you have to provide some evidence of likelihood as well as some evidence of causality. This has not been offered, either in general or in relation to the Knox case in particular.
What percentage of
all confessions are false, or "induced", or otherwise erroneous or misleading to the detriment of an innocent defendant? Can you answer this question? Can you provide some evidence from some reviewable authority? Is it 1%, or 10%, or 90%, or 0.0000001%?
Without such data for comparison no litany of tragic error or deliberate malfeasance, no matter how long the list may be, is anything other than a blatant appeal to emotion.