Final answer:
Keller et al found differences in classification of an independent group due to various reasons such as cultural influences and course choices. Controlling these factors is important to draw scientifically sound conclusions.
Step-by-step explanation:
The research conducted by Keller et al found differences in classification of an independent group. They mentioned several reasons for these differences. One reason could be the influence of cultural systems, as boys and girls might have been encouraged differently by their families and teachers to pursue certain subjects. Another reason could be the difference in courses taken, where boys might have taken more math courses and girls might have taken more English courses. These factors, along with others, need to be controlled in a study design to draw scientifically sound conclusions.