This paper reports results of an assessment of e-learning materials with the INTERFACE software evaluation methodology. On the one hand, this method of analysis allows us to identify ergonomics problems; on the other, to decide to what extent those problems and their severity concern all users in general and to what extent they depend on the users? type and characteristics. This is the first publication to apply the new marking, export, and statistical features of INTERFACE used in a quantitative analysis of heart rate variability (HRV) curves instead of earlier time-based statistics and qualitative methods. It presents correlations between event-related characteristics of human-computer interaction and the currently required mental effort showed with HRV. The paper also discusses correlations between variables and cognitive-style test scores which indicate the role of individual differences in ergonomics.