The present study was aimed to reconsider the structure of the QEWB with a wide range of age in Japanese sample using an exploratory approach. Based on goodness-of-fit indices and interpretability, we found a higher possibility that the scale should consist of not one or six factors, but actually consists with three to five factors. More specifically, our results suggested that a four-factor structure was most appropriate for individuals aged between 18 and 29 years, while a three-factor structure was best for individuals ranging between 30–49 and 50–69.
Regarding the original QEWB, Waterman et al.  first presumed the existence of six interrelated categories, including (1) self-discovery, (2) perceived development one’s best potentials, (3) a sense of purpose and meaning in life, (4) investment of significant effort in pursuit of excellence, (5) intense involvement in activities, and (6) enjoyment of activities as personally expressive. Further, the researchers conducted a pilot study indicating that the scale was unidimensional. This was later disputed, however. Primarily based on goodness-of-fit indices, Fadda et al. [8, 11] claimed that either a three-factor or bifactor three-factor structure may be best in terms of understanding the QEWB. Note that the fit of bifactor three factor model is the same as four factor ESEM model because the two models would show exactly the same likelihood. This emphasizes the need to consider interpretability when determining the appropriateness of bifactor structures. Contrary to the previous studies listed above, this study found that a four-factor model was best for the 10s to 20s, while a three-factor model was best for both the 30s to 40s and 50s to 60s. These results may enable a better interpretation of the QEWB. Indeed, our results are congruent with Schutte et al. , who also reported that three or four factors would better explain the data (Additional file 2).
Despite Joshanloo’s  warnings about the use of Western eudaimonistic models and measures in Eastern cultures, the results of this study, Schutte et al. , and Fadda et al. [8, 11] suggest that Sense of Purpose and Effortful Engagement consistently appear in the QEWB across cultures and age groups. While perhaps less robust, Purposeful Personal Expressiveness was also supported in these studies. Following this evidence, all three factors may be central aspects of eudaimonic well-being, which therefore robustly appear in almost every study.
In this study, Deep and Meaningful Engagement was newly presented in the 10s to 20s. Notably, this factor included all six categories originally mentioned in Waterman et al. . It was also moderately correlated with the other factors, thus indicating a more general aspect of eudaimonic well-being. Nevertheless, the highest factor loading (0.687) was found for the item ‘I find I get intensely involved in many of the things I do each day,’ which may especially be reflected by and/or serve as an expanded version of what Waterman et al.  (2010) called the Intense Involvement in Activities category. While Waterman et al.  particularly focused on the intensity of one’s commitment to activities when describing this category, this factor implies more that the activities are personally expressive (item 17), feel right for one to engage in (item 14), and involve one’s best potentials (item 15). Thus, while Effortful Engagement factor reflects the willingness to put effort into matters regardless of difficulty , this factor weighs more on the ‘flow’ and enthusiastic attitude one may have toward those activities.
Deep and Meaningful Engagement may have only appeared in the 10s to 20s due to variations in how people of different ages experience the QEWB factors. First, it must be noted that the items included in Deep and Meaningful Engagement did not dissolve in the 30s to 40s and 50s to 60s, but were instead included in other factors (e.g., Sense of Purpose). Second, although we mentioned above that Sense of Purpose, Effortful Engagement, and Purposeful Personal Expressiveness appeared in all age groups, we cannot disregard the fact that there were intergroup differences between the items included in each factor. For example, Sense of Purpose only contained five items in the 10s to 20s, but contained 11 and 10 in the 30s to 40s and 50s to 60s, respectively. Further, many of the items included in Deep and Meaningful Engagement were also found in Sense of Purpose in the 30s to 40s and 50s to 60s. Although all respondents aged 20–69 seemed to experience similar eudaimonic aspects, these results may indicate differences in the way they are experienced. In other words, older people may have a broader scope of Sense of Purpose than younger people, thus creating a new, broader factor in the 10s to 20s.
In addition, results may heavily reflect the statistical methods and scale development procedures used in this and previous studies. In fact, we reported the factor structure results obtained from rotations aimed for a simple structure. However, the complexity of item wordings resulted in many multiple factor loadings, which prevented us from reporting simple structures. Schutte et al.  also pointed out that Effortful Engagement factor may be the result of a methodological issue, since all items included in the factor were reverse-phrased. Further research is therefore needed to determine the most appropriate methodology for examining the concept of eudaimonia.
It is also important to remember that the goodness-of-fit indices were inconsistent, which may suggest the need to reconsider the scale at the item level. In other words, we should reevaluate whether the 21 scale items are fully adequate for capturing the concept of eudaimonic well-being. For example, as Waterman et al.  claimed, ‘the process of self-discovery may be central to eudaimonic functioning.’ However, when looking at items that should reflect this idea (e.g., ‘I can say that I have found my purpose in life’ and ‘I believe I know what I was meant to do in life’), self-discovery is not depicted as a ‘process,’ but as a ‘completed’ state. As being in the process of finding one’s purpose and potentials in life is central to this concept, it seems necessary to include items that reflect it.
In terms of future research on scale development, we should note that while we did not produce evidence of cultural differences in how eudaimonic well-being is perceived, it would be premature to conclude that none exist. For example, the QEWB does not include items that reflect fulfillment through interpersonal relationships, despite this being an important aspect of happiness in East Asia. Also, the more negative aspect of well-being  and mere appreciation of being  should may be reflected in the items. It may therefore be necessary to reexamine the concept from an Eastern eudaimonic perspective . This should entail not only the comparison between Western and Eastern viewpoints, but discussion on what the essence of eudaimonic well-being is, regardless of cultural differences.
Taking these perspectives into account, additional research may be required to develop a scale that substantially reflects the concept of eudaimonic well-being, particularly one that is not a methodological artifact and contains items of less complexity.
Limitations and future perspectives
This study has several limitations. First, we did not fully examine the relationship between the QEWB and external variables, more specifically in regard to convergent validity. Additional research should therefore investigate this point. Secondly, although it was not a key objective to investigate developmental changes in the experience of eudaimonic well-being, it should be noted that this study employed a cross-sectional design. Considering the wide variation in participant ages, future studies should also examine developmental changes in the experience of eudaimonic well-being. This developmental change mentioned here is not just limited to the change of the scale score value on common scale but to incorporate change of the construct of eudaimonic well-being. Our study indicated such construct level difference across age groups should be reconsidered.
Despite these limitations, this was the first study to investigate the structure of the QEWB among a wide age range of Japanese sample. It was also the first to consider Deep and Meaningful Engagement. Our findings are particularly valuable because we adopted an exploratory approach to examine the appropriateness of the one-to-six factor structure originally suggested by Waterman et al. , thus eliminating the potential biases of the researchers. However, continued research is needed to further investigate the concept of eudaimonic well-being from both the theoretical and empirical perspectives.
One future direction from the data analysis perspective may be to conduct multiple group analysis. This study aimed to re-consider the structure of QEWB questionnaire for each age group and the data analysis was separately conducted for this purpose. To test equivalency of factor loading across age group, multiple group SEM analysis is required. Another possibility of data analysis is combining all age group as one sample and conducting EFA, which we did not do in this study due to different research purpose. This might provide a clue of a general eudaimonic well-being scale across age group. If such general scale could be established, the quantitative developmental change could be assessed. We opened the data employed in this study to allow re-analysis to be freely conducted.
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