My introduction to the world of being a field applications engineer was a frantic call from a sales person. His customer had been told that a competitor's oscilloscope was "lower noise" due to Faraday cage shielding, something that presumably my product did not have. Now, I recognize marketing hype when I hear it, but I had to understand the misperception in detail to explain it to the customer. Every scope has "faraday cage shielding" - it's a standard part of designing high bandwidth front-ends. Michael Faraday died in 1867, so it amazed me that anybody would act like using a metal shield was something revolutionary, but here it was in marketing literature.
At first I suspected the customer was concerned a small difference in noise that really didn't matter. But when the customer said we had twice the noise of our competition, I had to understand what was going on.
How do you measure noise on an oscilloscope?