Frequently students measure incorrectly because they do not measure from the 0 mark of the ruler. With the numbered link students must be careful to begin their measurements at the 0" mark, and not at the end of the numbered link. Those who begin at the end of the link get results 1/2" less than others. Such a result is a strong indication of a measurement error.
We suggest using the numbered link to measure. When students use a standard ruler, they often try to measure the output distance to the nearest 1/4" or 1/8". Since there are always slight errors in moving the input exactly 1" (or 2" or 3") these errors result in measuring the output distance as 2 1/4" or 1 7/8" rather than 2". The numered link encourages estimation to the closest inch or half inch. This produces data that is much easier to interpret.