If you want to create a report that shows the average times in which tasks were completed, you can create a new report and choose
Y-axis: Completion times.
A task's completion time the duration between either the task's start date or (if a start date isn't set) the task's creation date, and the date when the task was completed. If a task is marked as complete multiple times, the last completion date will be used.
If you want to only include tasks with a start date (and exclude tasks that don't have a start date set), you can add the
Has a start date filter.
Y-axis: Completion times, you can use any other
Group by, and
Filter settings. For example, if you'd like to see the average completion times per assignee during the last month, you can use
X-axis: Assignees and add a
Completed > Within > Last 1 month filter.
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