While the Doctor's age has never been a known quantity, these numbers are the most difficult to reconcile with the rest of the series.

To get back a known quantity was too good to pass up.

For better or worse, those authors are a known quantity.

"Scott is a known quantity who has earned the opportunity to have this important role."

But a known quantity was gone, and an unknown had been introduced.

I'm a known quantity so I make them feel safe.

"He is a known quantity to the members of Congress, and they will feel comfortable with him."

These have been used on the exterior of homes for many years, he said, and they are now a "known quantity."

But if nothing else, something which has been around is a known quantity.

But then, Susan was a known quantity, so to speak.