Flip a penny and the chance of getting a tail (or head) is ½ or 50%. No problem-this is a concept that anyone can understand.
But change the scenario slightly. Suppose a penny held vertically on a table by the index finger of one hand is spun vigorously with a flick of the other index finger and allowed to come to rest flat on the table. Is the probability still 50% of either a head or a tail facing up?
Experts refer to this phenomenon as the "pop bottle cap effect". Find a cap from an old 16 oz. bottle of Pepsi or Coke and spin it in the same fashion we did the penny. About 90% of the time or more, the cap will fall with its top facing down and the sides facing up. Now how does this relate to the penny? If you examine closely a relatively new shiny penny, you will observe that the edge around the penny protrudes further on the tail's side than on the head's side. Thus, the extra edge on the tail's side simulates the side of the pop bottle cap although certainly not as pronounced visibly. The experts proclaim that the extra edge produces results that in the long run converge on 60% tails facing up. Of course, if you use a worn penny, this advantage in favor of tails disappears. Amazing but true!
MORALE OF THE STORY : It is up to you Quality people to have the technical knowledge to know the “USE AT THE CUSTOMER” and “THE PRODUCT SPECS” to be able to know about differences like this!