How to arrange shifts for 9 people?

The 9-person double shift system is actually to divide 9 people into two shifts. Because 9 is an odd number, it can't be evenly distributed in two classes, so we have to adopt the rotation system. Here is the solution. First, divide eight people into two groups. The rest is to be determined. The first and second shifts are on duty first. In the second shift, insert the remaining one into the first group and the second group respectively. In this way, one person in each group takes turns to rest.