Having been translated into more than 27 languages, CLES+T is now used in over 40 countries . In this study we propose the Polish version of this research instrument, as it displays good validity and reliability levels and can be recommended for use in the evaluation of CLE, supervision, and the role of nurse teacher in postgraduate nursing education. Cronbach’s alpha for the Polish version of the scale was 0.97. Cronbach’s alpha coefficient for the CLES+T sub-dimensions ranged from 0.83 to 0.95. These results are in line with other validation studies performed in culturally similar environments. For example, in the Slovak version, Cronbach’s alpha of the 34-item CLES+T was 0.94, and Cronbach’s alphas for each of the five subscales ranged from 0.80 to 0.97 . In the Croatian version, the overall Cronbach’s alpha of 33 items was 0.97, and Cronbach’s alphas for each of the subscales ranged from 0.77 to 0.96 . In the Slovenian version, the overall Cronbach’s alpha of 34 items was 0.96, and Cronbach’s alphas for the subscales ranged from 0.78 to 0.94 . However, it should be underlined that all of the previous studies took place in an undergraduate education context.
In our own research, a five-factor structure was confirmed in accordance with the assumptions adopted by the authors of the original version of the scale. The five-factor model with the explanation of 67.9% of the total variance in our study was comparable to the factorial models of other studies in Europe. To provide some examples – in the original, Finnish version: 64% of the total variance , in the Spanish version: 66.4% of the total variance , in the Cypriot version: 67.4% of the total variance , and in the Slovenian version: 67.69% of the total variance . The four-factor model of CLES+T resulted from Croatian validation study and explained 71.5% of the total variance , in Austrian studies with explanation of 73% of the total variance , and in German studies with explanation of 72.85% of the total variance .
The strongest factor in studies on the Polish version of CLES+T was the sub-dimension the content of supervisory relationship with high loading from 0.741 to 0.891, explaining 51.1% of variance. The sub-dimension role of the nurse teacher was found to be the second strongest factor in our study, with loading from 0.337 to 0.905 and the explanation of 5.8% of variance. This is in line with the Cypriot study in which this factor explained 11.02% of variance .
Considering these two factors as being the strongest in our study, we can assume that both, the internship supervisor (clinical mentor) and course coordinator (nurse teacher), are essential in successful clinical postgraduate education. Their high position is related to the scope of their responsibility. In the case of internship supervisor, it is mentoring as well as evaluating students’ achievements in clinical setting, while in the case of course coordinator, it is organising the whole training; selecting teaching staff, evaluating practical training institutions, being available and assisting in solving problems of postgraduate students . The mentorship approach and the role of nurse teacher are acknowledged in undergraduate nursing education [19; 37], and this is visible in other validation studies of CLES+T in which these factors were also strong [7, 34, 35, 36, 38, 39].
The weakest factor in this study was the leadership style of the ward manager (3.2% of variance explained), a result similar to that in the Cypriot studies  and opposite to the German studies in which this factor was the second strongest . In other studies, the weakest factor was usually nursing care on the ward [7, 34, 39]. Considering the weak results of the leadership style of the ward manager factor in our study, the possible explanation can lie in the working experience of postgraduate study participants as registered nurses (in our case average work experience of nurses was nearly 13 years) and thus their realistic evaluation of management style of ward manager and also his/her role in postgraduate clinical education. In undergraduate nursing education, Pitkänen and colleagues  indicated that students nurses assessed lower the management style of the ward manager than students from other healthcare programs, which may show that future nurses recognise the role of ward manager in creating educational atmosphere and assess it in a more reflective way.
The individual item means in our study varied between 4.24 and 4.79 and can be considered as relatively high, especially when compared with Swedish results where the individual item means ranged from 2.4 to 4.4 . The mean results for individual subscales in Polish research were also calculated, as follows: for pedagogical atmosphere: 4.56±0.54, for leadership style of the ward manager: 4.58±0.65, for nursing care on the ward: 4.52± 0.63 (the lowest), for the content of supervisory relationship: 4.70±0.53, and for role of the nurse teacher: 4.73±0.45 (the highest). These results show that the work of educators responsible for the entire clinical training process (an internship supervisor and a postgraduate course coordinator) is duly recognised and acknowledged. In a Spanish study among nursing students, the means for all subscales were generally lower than in our study. The lowest mean was calculated also for the subscale nursing care on the ward: 3.38±1.01, while the highest for pedagogical atmosphere: 4.15±0.63 . At the same time, in a Finnish study involving 1973 healthcare students, similarly to our results, the means for subscales were relatively high, although still lower than in the Polish one. As opposed to our study, however, the highest mean was calculated for nursing care on the ward: 4.56±0.55 and the lowest for role of the nurse teacher: 3.98±0.87 . It is difficult to explain the high results in our study for each subscale of the CLES+T. This is specifically interesting, because our respondents were mature students with previous educational experience, who, presumably, have certain expectations regarding the organisation and course of training. What is important and may contribute to a high rating, especially for the subscales pertaining to supervisory relationship and the role of nurse teacher, is that every centre for postgraduate education of nurses and midwives is obliged by law to have quality assurance systems in place. Therefore, every internship supervisor (clinical mentor) and postgraduate course coordinator (nurse teacher) are regularly assessed by students and, depending, on the results of such assessments, the centre decides whether to continue cooperation with them or not .
This study has its limitations, the main one being the convenience sampling method. Nurses taking part in the study were recruited from only one region of Poland and one educational institution which, in the case of our study, was a postgraduate education centre for nurses and midwives. Therefore, the sample of nurses was too homogenous to allow the formulation of conclusions of a more general nature. Research in a more diverse group of respondents is recommended. Additionally, the test-retest reliability of the scale – an important measure for the scale development – was not assessed in our study and it is recommended for future research.