Before scanners and high tech computers, a friend of mine ran a consumer survey. The questions were all printed on 3x5-inch index cards with the same questions printed on both sides. One side of the card had the standard blue lines and the other side was blank. The interviewer just flipped the card over and that with the first cut on segregating the “A” cohort from the “B” cohort. A few people asked to see the cards and no one ever figured out the trick was out in the open for all to see.
Krazy!