What I tell you three times is true
- Lewis Carroll

In DeMarco and Lister's classic book Peopleware, on page 53, they discuss unfounded claims that open-plan offices improve productivity:

The people who brought us open-plan seating simply weren't up to the task. But they talked a good game. They sidestepped the issue of whether productivity might go down by asserting very loudly that the new office arrangement would cause productivity to go up, and up a lot, by as much as three hundred percent. ... The only method we have ever seen used to confirm claims that the open plan improves productivity is proof by repeated assertion.
I have no idea if DeMarco and Lister originated the wonderful phrase "proof by repeated assertion", but they certainly helped popularise it in geek circles.

Proof by repeated assertion is repeating something over and over, without any actual proof, until everyone accepts it as fact. Repeated assertion of a proposition does not in itself make that proposition false, but it doesn't make it true either. If someone depends upon repeated assertion to the exclusion of real evidence, they may well be lacking any.

Here's an example: "Analogue vs Digital is not even an argument. One thing that is fact is that music that is recorded to analogue tape and then pressed to vinyl sounds better. No question. Digital is just not ready yet."

Proof by repeated assertion is an invaluable tool of propaganda. For political propaganda, you need mindshare in order for the debate to be framed on your terms. Getting people to believe the idea, for example, that invading and occupying another country (because they clearly have weapons of mass destruction) will reduce terror attacks, is the kind of big lie that can only be done by brazen repetition.

Insisting, say, that a particular group of detainees are bad people deserving of punishment, while repeatedly refusing to bring them to any kind of trial, is using repeated assertion of their wrongdoing as proof of it.

Repeated assertion is a key idea in advertising:

Most companies, when they run an advertising campaign, simply take the most unfortunate truth about their company, turn it upside down ("lie"), and drill that lie home. Let's call it "proof by repeated assertion." For example, plane travel is cramped and uncomfortable and airline employees are rude and unpleasant, indeed the whole commercial air system is designed as a means of torture. So almost all airline ads are going to be about how comfortable and pleasant it is to fly and how pampered you will be every step of the way.
- Joel Spolsky

Proof by repeated assertion is important in technology. Establishing your product as the "industry standard" and the competing vendor's offering as a "proprietary offering" when both are actually quite similar, requires repeated assertion.

See also