Strengths and limitations
The main strength of this study is the extensive preparation (qualitative study) in order to correctly identify the attributes of importance. Identifying the attributes is seen as the key aspect of the design of a DCE. Attributes should be relevant to the target population, should be relevant to the decision making context and the research question and the attributes should be defined in a way that is intelligible for patients. Without extensive work in defining the attributes, a DCE will not provide valid preferences, while important attributes could be missed that would have been paramount in making a treatment choice. With our qualitative, quantitative and literature search we believe that we used all possible information sources for an optimal selection of relevant attributes, and therefore increased the chance of valid preferences.
The second strength of this study is the strict selection of appropriate patients (26.3%) who were suitable for our DCE and would comply to our research question. Completing a DCE is a complex task and requires concentration and a certain level of intellect. With the three inclusion questions we aimed to select only patients with DE and bowel endometriosis, were not in actual fertility treatment and prior to a treatment decision. Furthermore, we excluded patients with poor health literacy, patients who filled in the DCE unreasonable quickly, patients who showed signs of being survey fatigue by filling in the same answer on almost all questions and finally we excluded patients who did not complete the DCE part of the survey. This way we aimed to select patients without linguistic bias, were focused, and took the time to fill in this DCE.
This study has two main limitations. The first limitation is the method of recruitment. The vast majority of patients (70.4%) were recruited by their gynaecologist and 29.6% through advertisement via the Dutch and Belgian endometriosis foundation. Ideally we only included patients via their gynaecologist who also confirmed the deep endometriosis with bowel involvement, but because deep endometriosis is less prevalent (≈ 1/10 of the general endometriosis population), we anticipated on the fact that recruitment would not be sufficient if we only included patients through their treating gynaecologists. We specifically choose advertising via official endometriosis organisations to reduce the potential bias recruiting patients without endometriosis, although there was no actual benefit in completing this DCE (no financial fee). To reduce the potential bias of patients without deep endometriosis we included three inclusion questions, whereby it was not clear for the participant which question would in or exclude the participant. This method of inclusion had been used more often, also in endometriosis research (15) (16) (17).
The second limitation could be the sample size. Prior to the start of the DCE we aimed to include 300 inclusions. We did not reach that amount of inclusions because recruitment took more time as previously anticipated. We took several steps to increase the number of participants like advertisement through the Dutch and Belgian endometriosis population. Finally, we were able to include 169 patients. Lancsar (26) states that based on empirical experience per questionnaire version rarely more than 20 respondents are needed although that number has to increase when performing extensive subgroup analyses. Given that we used three blocks (questionnaire versions) and performed one subgroup analysis, we calculated that our sample size of 169 was sufficient for reliable analysis.