6.4 Study design and variation in the response
Many aspects of the study design can influence the value of the response variable. Good design principles can be used to minimise the impact of these as much as possible, so the focus is on the influence of the explanatory variable on the response.
Example 6.6 (Design) The typing-speed study (Example 5.4) could be poorly designed.
For example, if females were always asked to use their dominant hand, and males always asked to use their non-dominant hand, the comparison would not be equivalent for females and males.Females would probably have a faster average time, simply because they are using their dominant hands.