In the left hand photo above, note that the 0” mark of the input is carefully aligned with the left side of the base. On the output, the yellow marker is aligned with its right side lined up with the left side of the base. This is the starting position for all three measures.
In the right hand photo the 1” mark on the input has been carefully lined up with the left edge of the base. The output moved a distance equal to that from the right hand edge of the yellow marker back to the left edge of the base. Each time marks are lined up with the edge of the base, students must be looking straight down, not at an angle from the side. Otherwise error is introduced.
In these experiments the output moves two and three times the distance moved by the input. Thus an error in measuring the distance moved by the input (not starting exactly at 0" or ending at exactly 1", 2" or 3") results in an error two or three times as big in measuring the distance moved by the output.