This study examines the effectiveness of inquiry-based instructional labs in improving engineering students' non-technical professional skills and attitudes to engineering by answering two sets of research questions: 1) Do inquiry-based labs enhance students' professional skills and attitudes towards engineering? 2) How does the complexity of learning tasks influence student learning outcomes in inquiry-based labs? This study adopted a multiple-case design to address research questions in naturalistic settings and a mixed-method approach with emphasis on the quantitative component. The final cases chosen included three lab courses offered at two large research universities in the Midwest: Engineering Design Lab, Chemistry Lab, and Bio-system Lab. To address if students' professional skills and attitudes towards engineering changed after inquiry-based lab learning experience, individual level survey data were collected from students enrolled in the three sample courses, using the Inquiry-based Lab Learning Inventory (ILLI) developed for the current study. Additionally, I interviewed seven students who had taken the Engineering Design Lab within five years. Paired sample tests were conducted to test the null hypothesis that students' skills and attitudes to engineering did not change before and after the sample course. Then, estimates of effect sizes and confidence intervals were employed to measure the magnitude of change. The second research question was addressed through quantitative analysis of the survey data. Two survey items asked about students' perceived difficulty and workload of the course. Multivariate regression analysis and separate regression analyses were performed to examine the association between difficulty and workload of the course and student learning outcomes. Across sample cases, students showed positive improvement in self-perceived skills in conducting lab work, attitude to teamwork and communication, and using active coping strategies when facing stress or difficulty in problem solving. The magnitude of change varied across sample cases. Students taking the Engineering Design Lab showed greatest improvement in these areas. Additionally, regression analyses results showed a positive association between perceived difficulty of the course and students' attitudes to teamwork and communication and coping strategies. However, the benefit of increased difficulty on these two learning outcomes started to decline after an upper middle difficult level. Therefore, excessive difficulty of a lab experiment of project could compromise the benefit of inquiry-based learning. Findings about how inquiry-based instructional labs influence students' self-perceived skills in conducting cooperative inquiry-based projects and attitudes to engineering have several implications. 1) Inquiry-based lab instruction is a potentially powerful instructional method to develop students' attributes of engineering professionals. It can be utilized as a complement to enhance the current engineering curriculum to achieve a balance between cognitive and affective learning. 2) Engineering programs should engage students in inquiry-based lab work early on rather than only in upper division courses. 3) Faculty, staff, or teaching assistants who design, oversee, or facilitate instructional labs should be provided with teaching supports, so that they will have the capacity and confidence to engage students in inquiry-based lab work. 4) Facilitators of inquiry-based labs should check students' perceived difficulty of learning tasks periodically and adjust the complexity of learning tasks or provide scaffolding accordingly to achieve better learning outcomes.
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