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.