The types of programming errors that novice programmers make and struggle to resolve have long been of interest to researchers. Various past studies have analyzed the frequency of compiler diagnostic messages. This information, however, does not have a direct correlation to the types of errors students make, due to the inaccuracy and imprecision of diagnostic messages. Furthermore, few attempts have been made to determine the severity of different kinds of errors in terms other than frequency of occurrence. Previously, we developed a method for meaningful categorization of errors, and produced a frequency distribution of these error categories; in this article, we extend the previous method to also make a determination of error difficulty, in order to give a better measurement of the overall severity of different kinds of errors. An error category hierarchy was developed and validated, and errors in snapshots of students source code were categorized accordingly. The result is a frequency table of logical error categories rather than diagnostic messages. Resolution time for each of the analyzed errors was calculated, and the average resolution time for each category of error was determined; this defines an error difficulty score. The combination of frequency and difficulty allow us to identify the types of error that are most problematic for novice programmers. The results show that ranking errors by severity--a product of frequency and difficulty--yields a significantly different ordering than ranking them by frequency alone, indicating that error frequency by itself may not be a suitable indicator for which errors are actually the most problematic for students.