## 2 Literature Review
### Warehouse Performance Indicators  
Warehousing is one of the most debated topics in logistics research [7; 15]. Warehouses are strategic supply chain nodes where goods are stored until released [16]. In particular, modern warehouses are complex technical structures which allow for high profitability [17]. In addition, they contribute to carbon footprints, and the technologies included in the warehousing activity have recently emerged as game-changing tools [15] since they can have a big impact on warehouse performance [18]. Among the topics discussed in the literature,an increasing interest has been revealed in the warehouse performance assessment [7], with particular reference to operational performance and its metrics [19], given its positive relationship with 3 PLs' financial performance [12].  
Due to the increasing growth of the logistics service market [1], measuring warehouse performance has become critical for 3 PL providers to increase or maintain their competitive advantage [7]. However, the quality of services has remained the key to winning over the customer [20]. In literature, it is possible to find interesting classifications for warehouse indicators [21]. The researchers distinguished direct indicators (i.e., quantitative measures of warehouse performance provided by a simple, straightforward mathematical equation) from indirect indicators, which are qualitative measures of warehouse performance (e.g., customer perception) and also involve other logistics activities (e.g., transportation) [21]. Among the direct performance indicators, they identify four main categories:  
* time: this category groups all the time metrics on the logistics activities performed inside the warehouse (e.g., delivery lead time);
* quality: this category includes all the elements related to the quality of the activity served (e.g., on-time delivery, accuracy);
* cost: these indicators measure the cost associated with the warehousing activities (e.g., order processing cost);
* productivity: it includes all the measures that define the level of asset utilisation; in this case, the asset considered is the warehouse (e.g., surface utilisation).  
In the indirect indicators, it is possible to find some of the items dear to the service quality stream of research, such as flexibility, proactivity, customer perception, etc. In the work of [12], indirect and direct indicators are put together. The operational performance (analysed from both provider and customer perspectives) is measured considering five key items: delivery (e.g., order accuracy), quality (e.g., order damage, customer care), flexibility (e.g., delivery of special order), cost (e.g., overall operating cost), and innovation (e.g., level of new value-added content of services). In the work of [14], he presents a list of performance indicators to measure the logistics service. Among them, several refer to warehousing activities, such as productivity indicators (e.g., surface and volume utilisation) and quality indicators (e.g., completeness, correctness, harmfulness, and lead time). As stated by [21], the quality indicators are customer-oriented measures that remain the most used in the literature, especially the literature on service quality measurement (see, for example, the SERVQUAL model by [22], the logistics service quality (LSQ) model by [23], and the logistics service measurement model by [14]). In Table 1, we report the warehouse performance found in the literature, classifying them through the framework proposed by [21].