Sally Jordan of the OU has put up some interesting graphs showing statistics on the level of difficulty of the same question, but in different format: Multiple-choice or Free-text.
Her graphs show that the free-text format is more easy than the other formats. She wonders how that comes about. It could be that in de Multiple-choice format, good distractors are present that are appealing for students to select.
Maybe she will put up some discrimination indices as well to give some insight in this.
Click on the link below for her article.