Often, it is difficult to get all your students in a room at the same time to take a quiz online, usually because the computer lab is not big enough for everyone. One way to deal with this is to create two copies of the same quiz and set one for one group of students with a particular start and end time and the other for the other group of students with its own start and end time.
Although this can work perfectly well, it begins to get messy if you want to make a change to the quiz because now you have two copies you have to remember to change.
Enter the Group Override. You can set a single quiz for your course and using group overrides, and even user overrides, specify open and close times and permissions for the quiz by group.
To do this, all you need to do is create your quiz as usual. Then, in the Quiz Administration block, you will see there is an option for Group Overrides. Click this and add a new Group Override. Complete the required fields for the override, and then you can either Save, or Save and Create Another Group Override. When you have completed all the group overrides you need, you can leave the screen. Now your quiz will obey the group overrides you have set and open and close for each group based on those overrides. And you only have one quiz to tweak when it is time to make changes.