Quiz questions can be randomly selected from categories and subcategories in the Moodle Question Bank.
Two categories within your question bank would need to be created that could be named 1st half & 2nd half. You then would add questions to each category. When you edit the quiz, you would select the number of random questions that you would like to add from the first half category and then the number of questions you would like to add from the 2nd half category.
Another way you might want to use categories is to create a category for each of the chapters in the book. You can then add random questions by chapter. When you add questions to the test, you can set the number of random questions to be added from each chapter category.
For readers who are not familiar with using quiz categories and the random option here are sample steps and screenshots to create a quiz with randomized questions:
Add the quiz activity to the course or select an existing quiz in your course and click on the link to the quiz in your course. When the quiz opens, select Edit Quiz from the Quiz Administration category on the Settings (Administration in 2.5) block:
Once you have selected Edit Quiz you will be able to add/delete questions to your quiz. To add a single random question, click on the Add a random question button located immediately under a question that has already been added to the quiz or where questions will be added in a brand-new quiz.
When you have clicked on Add a random question, you will see a pop-up window display. Select the quiz category you want the random question to be drawn from and click on Add a random question.
You will now see that a random question has been added to the quiz. The question also has the random icon and label of Random question:
If you want to add many random questions or add multiple random questions at one time there is a more efficient method. On the Edit quiz page select the correct category for random questions in the Question bank that is displayed. At the bottom of the question bank you will see the option to select how many random questions you would like to be added to the quiz. (Note: In this screenshot no questions have been previously added to the quiz.)
In the next example the choice was made to select three questions from the “About Moodle Part 1 Category.”
As you can see it is easy to have Moodle randomly select quiz questions from categories and subcategories of questions stored in the question bank for your course.