For Questions Below Use the Following: a Researcher Suspects

Short Answer

For questions below use the following: A researcher suspects that increasing illumination increases motoric speed. A group of 7 subjects was randomly selected and tested on the Digit Symbol subtest of the WAIS-R IQ test. The same subjects were tested under three illumination conditions, low light, medium light and high light. Their scores under each condition were as follows: $$\begin{array} { c c c c } \text { Subject \# } & \text { Low Light } & \text { Medium Light } & \text { High Light } \\1 & 7 & 10 & 11 \\2 & 8 & 7 & 10 \\3 & 6 & 8 & 10 \\4 & 5 & 6 & 9 \\5 & 4 & 4 & 7 \\6 & 3 & 5 & 6 \\7 & 8 & 10 & 13\end{array}$$
-What is the major research error in this study?