What is the best way to handle absences? Everybody seems to have a different take on this. Let's look at the four most common approaches:

Part-Time Employees
In smaller organizations, this can be a great solution. It's cheaper than overtime, and it doesn't place a huge burden on the full-time staff. The problem is finding and retaining good part-time employees. It is usually best for jobs that don't require extensive training or specialized skill sets. The exceptions to this are nursing and on-demand transit, as many organizations in these fields are able to find a large pool of candidates seeking part-time employment.

Overtime
The most efficient way to cover absences is by using overtime. Your existing staff is trained and readily available. Best of all, you only pay for the coverage when it is needed. On 8-hour shifts, you can split the vacant shift into two 4-hour sessions. Hold someone from the prior shift over for the first session and call someone from the following shift in early to work the second 4-hour session. 10-hour and 12-hour shifts are different. With them, you should call someone into work who is scheduled to be off. Most groups seek volunteers first before resorting to mandatory call-ins of the least tenured staff. If the overtime levels exceed 15% of the normal working hours on a regular basis, however, that is too high and you probably should hire more staff.

Relief Coverage
Time can be built into the work schedule for relief coverage. For example, if you need eight employees to provide the basic coverage, you could employ one extra person. You would then create a 9-week schedule for the group in which one of the 9 weeks is devoted entirely to relief coverage. Since all 9 people rotate through this 9-week schedule, they would equally share in the responsibility. This works best with rotating shift schedules since the relief person can cover any shift. Larger organizations often do this by adding a fifth crew. The extra capacity could be used for both relief coverage and training purposes.

Additional Staff
This is the most expensive approach since you would pay for the extra employees even when they are not needed. For example, if you needed 4-person coverage at all times, you might hire enough staff to have 5-person coverage. Many law enforcement agencies use this approach because it is critical to maintain a minimum level of coverage. They also tend to have rather high absence rates.

 
Each of these approaches has its own advantages and disadvantages. They are listed here in order of cost (from lowest to highest), so I would probably start at the top and test each approach until you find the best one for your unique situation and requirements.