ordered factors

Consider the following data from the text Design and Analysis of Experiments, 7th ed. (Montgomery, 2009, Table 3.1). It has two variables: power and rate. power is a discrete setting on a tool used to etch circuits into a silicon wafer. There are four levels to choose from. rate is the distance etched measured in Angstroms per minute. (An Angstrom is one ten-billionth of a meter.) Of interest is how (or if) the power setting affects the etch rate.