Physics education researchers have called for a shift from traditional laboratory (lab) instruction to labs that focus on the development of experimental skills. Although research has demonstrated the efficacy of such reformed labs, much work remains to determine what these labs can and should teach students and how best to achieve these goals. This thesis presents research on two such aspects of lab instruction: student reasoning about measurement and uncertainty and how group work roles impact women's and men's experiences in lab courses. The first part of this thesis describes our research on student reasoning about uncertainty. Previous research has primarily focused on introductory (intro) students' procedural reasoning in a simple, classical experiment. We expanded on this prior work by studying ideas about uncertainty across types of reasoning, student academic levels, and question contexts. We developed a survey to probe students' procedural reasoning (given some data, what should the student do?), predictive reasoning (given some different data, what will happen?), and reasoning about sources of uncertainty. We observed that upper-level students discussed different sources of uncertainty when asked about classical and quantum mechanics in a decontextualized question (principles of theoretical physics in quantum and limitations of an experiment in classical) but focused primarily on limitations when asked about specific classical and quantum experiments. We also observed that intro and beyond-intro students answered similarly on some types of questions, such as in comparing two data sets with the same mean but different spreads or in identifying how a data distribution would change if more data were collected, but differently on other types of questions, such as in determining whether two data sets agree or in identifying how a data distribution would change if better data were collected. Collectively, these results illustrate that student reasoning about uncertainty is context-dependent and varies across the physics curriculum. Our work motivates future research to identify how different instructional approaches affect student reasoning about uncertainty. The second part of this thesis addresses how the roles that students take on in group work in lab courses affect their course outcomes, particularly for women. Previous work has shown that men and women tend to take on different roles in lab courses, which may lead to inequity in the learning environment. In our research, we first interviewed five women who had taken a project-based lab course to understand the experiences of women who take on manager or leader roles in labs. We used a practice-linked identity framework to understand how the manager and leader roles affected women's opportunities for engagement and identity development and found that these roles could have positive, neutral, or negative influences on women who took them on. We additionally used linear regression to model quantitatively the relationship between roles and course outcomes for both men and women. We found no relationship between taking on an equipment-handling, note-taking, data analysis, or manager role and students' lab self-efficacy, lab critical thinking, or final course grade. Taken together, these results motivate future research to more precisely probe the relationship between course outcomes and roles and other group work structures. These results also suggest that future group work instructional interventions should consider individual students' needs as well as strategies that will benefit the class as a whole.