Mantenimiento preventivo planificado (MPP).

One of the major issues to be considered when applying DEA approaches is the selection of appropriate inputs and outputs. As stated in Cooper et al. (2006), the inputs and outputs selected should reflect an interest of an expert so that including these variables into the

instance, one input may be measured as number of people whereas another input in the same analysis may be in monetary terms (Cooper et al., 2006).

Outputs: When DEA applications in agricultural sectors are reviewed, it is possible to identify some common inputs and outputs taken into consideration by the majority of the studies. On the output side, most common output used is the agriculturalproduction realised by the decision making units. This output is either in the form of monetary values or in the relevant forms of measurement that represent the physical amount produced. In several studies, the output is represented by total monetary value of the production in the form of relevant currency (Piot-Lepetit et al., 1997; Thiele and Brodersen, 1999; Brümmer, 2001; Lansink et al., 2002; Iráizoz et al., 2003; Rodríguez-Díaz et al., 2004; Cherchye an Van Puyenbroeck, 2007; Balcombe et al., 2008a; Adhikari and Bjorndal, 2011) or such as in the studies by Galanopoulos et al. (2006) and Bojnec and Latruffe (2009), revenues realized by the farm.

On the other hand, significant amount of studies consider the production in terms of measures representing the physical production such as kilograms or tonnes (Coelli et al., 2002; Wadud, 2003; Reig-Martínez and Picazo-Tadeo, 2004; Balcombe et al., 2008b; Luik et al., 2009; Odeck, 2009). Additionally, in some studies physical production per a unit of the land are considered as output (De Koeijer et al., 2002; Wossink and Denaux, 2006; Garcia and Shively, 2011). If the evaluated units are dairy farms, total milk production in litres or kilograms are mostly considered as outputs (Fraser and Cordina, 1999; Balcome et al., 2006). A more detailed discussion of considering outputs either in monetary or physical forms from a multi-product farm point of view can be found in Section 3.3.

Inputs: Various inputs have been included in agricultural efficiency evaluation studies with DEA depending on the issues covered and evaluation scope of the research. Land and labour are the variables that are considered in majority of the studies. Land is generally defined as

the utilized agricultural area and measured in hectares or homologous measures. Labour is measured by different means such as number of workers (Grazhdaninova and Lerman, 2005), labour costs (i.e. wages) (Färe et al., 1997; Kleinhanß et al., 2007; Artukoglu et al., 2010), annual working units (Piot-Lepetit et al., 1997; Latruffe et al., 2004; Reig-Martínez and Picazo-Tadeo, 2004; Rodríguez-Díaz et al., 2004; Davidova and Latruffe, 2007; Balcombe et al., 2008a; Latruffe et al., 2008a) or labour hours (Fraser and Cordina, 1999; Reinhard et al., 2000; Lansink et al., 2002; Iráizoz et al., 2003; Asmild and Hougaard, 2006; Galanapoulos et al., 2006; Luik et al., 2009).

Naturally, costs are among the key input factors. Costs are taken into account through different means. On one hand, in many studies, costs are integrated into the models as an aggregated variable represented with different labels as ‘cultivation costs’ (Iráizoz et al., 2003), ‘intermediate consumption’ (Millian and Aldaz, 1998; Latruffe et al., 2004; Davidova and Latruffe, 2007; Balcombe et al., 2008a; Luik et al., 2009), ‘materials’ (Alene et al., 2006), ‘purchased inputs’ (Helfland and Levine, 2004; Adhikari and Bjorndal, 2011), ‘total expenses’ (Amores and Contreras, 2009), ‘variable inputs’ (Thiele and Brodersen, 1999; Reinhard et al., 2000; Cherchye and Van Puyenbroeck, 2007; Bojnec and Latruffe, 2009) or ‘other expenses’ (Mathijs and Vranken, 2000; Lansink et al., 2002). These aggregated variables can represent the sum of costs on various items in agricultural production process such as energy, fertilizer, feed, fuel, seed, machinery, pesticides, water or farming overheads. The aggregation of cost input (i.e. what is included into this item) varies between studies.

On the other hand, some costs or usage of various items in the agricultural production process are considered as separate inputs instead of taking them into account under the aggregated costs. Common examples of such variables are fertilizers (Jha et al., 2000; Kim,

pesticides (De Koeijer et al., 2002; Reig-Martínez and Picazo-Tadeo, 2004; Wossink and Denaux, 2006), seed (Piot-Lepetit et al., 1997; Dhungana et al., 2004; Balcombe et al., 2008b; Odeck, 2009) and energy consumption (Morrison Paul et al., 2004; Asmild and Hougaard, 2006; Bogetoft et al., 2007). In significant number of studies, these variables are taken as inputs themselves. Items such as fertilizers, seed and pesticides are represented either with monetary terms or the physical amount purchased.

Another important variable used as an input is the capital factor. It has been considered in different forms in several studies. One way undertaken by some studies is to incorporate the sum of depreciation of fixed assets and the interest payments as a capital factor (Latruffe et al., 2004; Latruffe et al., 2008b; Balcombe et al., 2008a). Another way is to relate capital factor to the machinery and other fixed capital such as hours of used machinery (Reig- Martínez and Picazo-Tadeo, 2004), annual costs on capital (Färe et al., 1997; Lilienfeld and Asmild, 2007; Townsend et al., 1998) or book value of machinery and inventory (Lansink et al., 2002; Iráizoz et al., 2003). The use of total assets (Brümmer, 2001; Bojnec and Latruffe, 2009), reported capital in balance sheet (Mathijs et al., 1999) and depreciated value of total assets (Davidova and Latruffe, 2007; Luik et al., 2009) are the other ways considered by the researchers to incorporate capital input into their models.

Inputs identified also vary depending on the product type of the units evaluated. In the research dealing with farms producing livestock or dairy products, it can be observed that animal related inputs are also taken into consideration. Common examples of these variables are number of animals (Fraser and Cordina, 1999; Balcombe et al., 2006) and feed (Sharma et al., 1999; Fraser and Cordina, 1999; Morrison Paul et al., 2004; Galanopoulos et al., 2006; Balcombe et al., 2006) either in terms of amount or expenditures made for it. Feed is usually considered as a separate variable, whereas in some studies it is included in aggregated costs as mentioned above.

Furthermore, the inputs identified for DEA studies in agriculture exhibit slight differences depending on the evaluation context of the study. For instance, in the studies dealing with environmental efficiency, some specific environmentally related inputs such as nitrogen, phosphorus or potassium surplus variables are used. (Reinhard et al., 2000; De Koeijer et al., 2002; 2003; Asmild and Hougaard, 2006). These types of variables are considered as inputs or undesirable outputs and aimed to be minimised, since they are environmentally detrimental factors. Another example can be given the studies dealing with the evaluation of irrigation efficiency. In these types of studies, it is inevitable to consider variables related to water. Rodríguez-Díaz (2004), in which the irrigation districts in Spain are assessed, water applied in each district is considered as an input variable. Similarly, Lilienfeld and Asmild (2007) take water use and precipitation in evaluating the irrigators in Kansas, USA. Recently, in an irrigation efficiency work by Frija et al. (2011), water use is considered as an input.