Their conclusions? Well, in a nutshell, there is a strong correlation between attendance at class and attainment, not just in terms of pass/fail but in actual grades. They also noted the much suspected dropping off of attendance as the semester progresses, noting spikes in turnout when revision or information about assessments were the topics under discussion. Early morning lectures weren't as much an issue as Friday afternoons were!
Details: " A large-scale investigation into the relationship between attendance and attainment: a study using an innovative, electronic attendance monitoring system", L. Newman-Ford, K. Fitzgibbon, S. Llyod, S. Thomas, Studies in Higher Education, Vol 33, No. 6, December 2008, 699-717