Variations in science lab design can differentially impact student learning. Quantification of these differential impacts can be used in modeling--an approach we term "optimal lab design." In this study we estimated relative influences of six characteristics of lab design on students' attitudes toward science labs in three different first-year college biology lab courses (USA). We used two end-of-semester surveys. The first had students choose their favorite and least favorite lab and answer questions associated with the six characteristics and their choices. The second had students provide an overall rating of each lab and a rating based on their perception of the degree to which the six characteristics impacted the lab. Results of the two assessments were similar and indicated the following: Total Student Attitude = 0.39 Exciting + 0.25 Time Efficient + 0.15 Not Difficult + 0.10 Lecture Help + 0.08 Experimental + 0.03 Open-Ended.