*5.1. Selection of Decision Variables in the Evaluation Process*

To assess the effectiveness of lean maintenance tools, the factors influencing the dependent variable were identified. In the research on the assessment of the effectiveness of lean maintenance tools, one dependent variable and 19 explanatory variables (predictors) were determined. In the conducted research, one dependent variable was assumed: the average value of the OEE indicator.

Due to the large variety (combination) of response options, three additional indicators were introduced in the surveyed enterprises: maintenance strategy indicator (MSI), number of preventive activities (NPA), and number of TPM activities (NTPMA) indicator. The NPA number is the number of actions to prevent unplanned downtime, calculated as the total value of actions carried out simultaneously by the enterprise. During the survey (data collection) process, the company could choose several activities from the following:


Depending on how many activities are carried out by the enterprise at the same time, the indicator may range from 1 to 10. In addition, during the survey (data collection) process, the company could choose several activities implemented as part of the implementation of the TPM method, recommended in the literature on the subject, from the following:


$$NTPMA = \frac{\sum\_{i=1}^{11} x\_i}{\maxnumber \, of \, activity} \* 100\% \tag{3}$$

Depending on the value obtained, the indicator had four levels: low, medium, high and very high (Table 3).



The last index developed is the MSI index. With this indicator, it is possible to determine what technical infrastructure management strategy is applied by the enterprise. During the study (data collection), the company could choose several activities defining the realized activities implemented under the corrective maintenance (CM), preventive maintenance (PM), and condition-based maintenance (CBM) strategies. In order to define the index for possible variants of answers, numerical values ranging from 1 to 7 were introduced (Table 4). The lowest value was given to the action implemented in accordance with the CM strategy as the least effective strategy. However, the highest efficiency (value 7) was adopted for the operation: continuous monitoring of the condition of all machines (e.g., noise, vibrations, temperature) (CBM).

**Table 4.** Maintenance strategy—realized activities.


The value of MSI indicator is calculated as the sum of the value of activities by the number of implemented activities (4).

$$MSI = \frac{\sum\_{i=1}^{n} x\_i}{n} \tag{4}$$

The MSI indicator may take values from 1 to 7. Value 1 means mainly the CM strategy, value 3.5—PM strategy, value 7—CBM strategy. When the value of the ratio is <3.5, it means the implementation of a mixed strategy, mainly CM–PM; when >3.5, it means the implementation of mainly a mixed strategy PM–CBM. At the same time, when closer to the value of 3.5, PM is the prevailing strategy. In order to ensure the adequacy of the adopted indicator, the variants of the strategy implemented by the examined enterprises were analyzed. For individual values of the indicator, implemented strategy variants (sequence of implemented actions) were assigned. The distribution of variants of the implemented strategies (distribution close to the normal distribution) allows us to confirm the validity of the adopted indicator (Table 5).

**Table 5.** The values of maintenance strategy indicator (MSI) indicator.


The Statistica Data Miner system was used to conduct the analyzes. This system enables the preparation of data in the form of a training and test set, intuitive guidance through the model building and fitting procedure, and a clear visualization of test results.
