**6. Limitations**

One limitation of this study was that the sample was not randomly selected, which precludes making generalisable statements about mentoring processes that include the immigrant population. This study was based on a single group, pre-test–post-test design, without a control group. The objective was to measure certain changes in the emotional well-being of young people, and it is therefore not an impact analysis of the project as it does not have a comparative reference in a group on which the activity has not been developed. One limitation that should be taken into account is the e ffect that other non-controlled variables may have had on the observed changes in some aspects of the emotional well-being of young people. The non-experimental pre-test–post-test design does not manage to control confounding variables that might exert some influence on the results. This limitation will be addressed in a second phase of the study, which will incorporate a qualitative analysis of interviews with mentees in which information will be collected about their perception of mentoring in their emotional states. The main di fficulty will be that of di fferentiating the specific e ffects of the programme from those non-specific e ffects that derive from the lack of comparability of the group with a control group, which compromises the internal validity of the study. Therefore, causal e ffects are not deduced from this analysis; rather, the di fferences in variability in scores need to be understood as possible e ffects derived from the intervention. In fact, this type of design is not exempt from the Hawthorne e ffect; that is, the responses may have been induced by the participants' knowledge that they are being studied.

Nor do the participants represent all the geographical areas where the Nightingale programme is implemented, which means that these results cannot be extrapolated to the mentees that take part in other universities around Europe (Sweden, Norway, Austria, Switzerland, etc.) and Africa (Ghana).

Furthermore, the use of self-report questionnaires structured on Likert-type scales made it di fficult to carry out a more precise evaluation, as the participants gave their own meanings and interpretations. While this method allowed a very wide range of e ffects to be explored—many of which were inaccessible to direct observation—in a relatively short time, the use of this methodology prevented the appearance of emerging categories on the nature of the relationships and their e ffects. To increase the validity of the measures, we recommend that future evaluations be conducted with multiple sources, for example gathering the testimonies of parents and teachers, and through the use of the multimethod perspective or

multiple approaches that facilitate the possibility of studying mentoring relationships quantitatively and qualitatively.

One additional limitation, which could explain the moderate effect of the programme, is the length of the study. Some research points out that the minimum time it takes to build a trusting relationship is six months [68]. It has also been shown that the longer and more trusting the mentoring relationship, the greater the impact capacity of the programmes [69].

Finally, it should be noted that for certain instruments used, the measurement value of their consistency was below the standard values, a fact that that must be taken into account when drawing conclusions, and that necessitates the continued improvement of the instruments applied to the case of mentoring.
