**2. Background and Related Studies**

### *2.1. Planned Preventive Maintenance (PPM) and Unplanned Maintenance (UPM)*

PPM and UPM are two well-established approaches in the facility maintenance domain. A study proposed that PPM is carefully prepared in advance as it is done at scheduled times and is expected to be very efficient [3]. PPM is also defined as pro-active, where planning and execution of maintenance work are carried out in anticipation of the failure of facility [10]. Another study speculated in their case study that PPM can reduce the demand for correction [11]. Preventive Maintenance (PM) and planned maintenance are two primary components comprising PPM [1]. PM is a type of facility maintenance that increases the reliability or lifespan of a building and equipment is performed through periodic inspection, lubrication, and minor replacements [1,4,12]. Planned maintenance is a pre-determined job procedure that documents labor, materials, tools, and equipment to perform the task before implementing maintenance work [1].

In contrast to PPM, UPM is the work performed as the direct result of equipment failure. Since equipment failure occurs randomly, controlling UPM occurrences is a challenging task. A study reviewed maintenance definition for maintenance, repair, and replacement (MR&R) types, where UPM includes service calls, emergency responses, and unanticipated tasks [5]. UPM is also defined as reactive or emergency maintenance which leads

to high maintenance costs [4]. Within UPM, reactive maintenance is a type of work done immediately after a failure to bring an asset back into operation [1]. Figure 1 illustrates the hierarchical structure of facility management as per operations & maintenance (O&M) defined by APPA [1].

**Figure 1.** Hierarchical structure of facility management [1].

As shown in Figure 1, annual resource allocation for facility management can be divided into four major maintenance categories: PPM, UPM, deferred, and non-maintenance. PPM consists of planned and preventive maintenance. Planned maintenance refers to scheduled and corrective maintenance, while preventive maintenance reflects routine and predictive maintenance. The corrective maintenance can be categorized under both planned and unplanned maintenance based on APPA [1]. However, corrective maintenance has been used differently by institutions or facility managers [13–15], this study considered the corrective to be unplanned maintenance. Reactive maintenance refers to emergency and unscheduled maintenance. Deferred Maintenance can be divided into many maintenance types, such as deferred corrective, recurring, backlogs, renewals, demolition, etc. [1,7]. The non-maintenance work orders include events, custodial work, warranty work, delivery and transportation of equipment and supplies, signage, banners, etc. This study also excluded project-based work orders (i.e., renovations) which involve contractors outside of the facility management.
