What did they want to find out with this study and why did they leave out Europe? Also: Did they consider the possibility that the test was biased since it "was

What did they want to find out with this study and why did they leave out Europe? Also: Did they consider the possibility that the test was biased since it "was developed by the U.S. Educational Testing Service"? I guess the exams that Chinese students get might be different from this one, while it is probably a familiar structure for US students...