Once I had a new L reg car which was 10 years old (new to me, not an oxymoron).
Just by naturally observing all the cars passing me (when on foot) it seemed that about half of them were an L reg.
Obviously it was impossible for half of all cars to be exactly 10 years old, so I started to pay attention to every car that passed: AN LREG, no, no, no, no, no, AN LREG, no, no, no.
This illustration shows how my brain noticed all the L reg cars and ignored the rest. I never did measure the actual frequency, I just established it was well below 50%.