There is great interest in understanding and influencing students' attraction to computing-related majors. This qualitative study is based on interviews with 31 students enrolled in introductory programming courses at two public universities in the United States. This paper presents a model of five factors that influence student decisions to major in CS and elaborates on our grounded theory analysis of one of these factors: how students assess their CS-related ability. We describe how students measure their ability in terms of speed, grades, and previous experience and how students make interpretations and decisions based upon these measurements. We found that students' interpretations were influenced by experiences in their environments and beliefs about ability as being fixed or malleable.