Studies based on response processes of individuals can provide information that supports the assessment and increases the validity of the items in the scale or tests. The purpose of this study is to present the extent to which the student response processes are effective in identifying and developing the characteristics of the items in an achievement test and in collecting validity evidence. For this purpose, 28 Turkish fourth-grade students were chosen, half were high-achieving students and the remaining half were low-achieving students. The items for the study were chosen from the Trends in International Mathematics and Science Study TIMSS 2007 and 2011 by taking into consideration several item characteristics. Before cognitive interviews, an interview guide was also prepared. In the study, it was determined that cognitive interviews, especially those conducted with the high-achieving students, can serve to develop item validity. In the cognitive interviews with the low-achieving students, information was gathered concerning how students who did not have specific knowledge measured with an item were able to respond to that item. [This study was presented at the 10th International Test Commission (ITC) Conference in 1-4 July 2016 in Vancouver, Canada.]