Quiz Statistics and Ungraded Attempts

The quizzes page shows the number of attempts and graded attempts for each quiz, separated by a slash. If the numbers are not equal (i.e., if there are one or more ungraded attempts) clicking on the statistics icon may show an error. You can mark a quiz attempt as graded by clicking on the grade icon for that quiz. Then click on any ungraded attempts to view them, click the Auto-Grade button, click the check box to indicate the quiz is Graded, and then click the Save button. The quiz statistics should be visible.

Even if the statistics icon returns an error, you can likely view the statistics for each question on the quiz and the overall statistics in the grade area (if the quiz is associated with a grade item).

Thanks to Eddie Fagin for asking the question that led to this answer.