## 4 Discussion
### Limitations of the Study  
Recruiting patients for a clinical study, who fulfill a set of inclusion criteria and retain their involvement during the follow-up measurements is a challenge. Together with the stratified analyses for the different headache types, the sample size of the subgroups (17 to 22 subjects) can be considered a limitation of the study. Still, the post hoc power calculations showed that 16 patients were sufficient to show a clinically relevant change in disability score. Researchers have been encouraged to also present clinical relevance analysis in order to simplify the transfer of knowledge from research into practice [35; 36].  
Many participants experienced more than one headache type. In the TMD pain and migraine group, 54.5% also experienced a probable TTH, and in the group with headache attributed to a TMD more than 70% experienced a probable migraine and/or probable TTH. In case a participant fulfilled the criteria of more than one type of headache, the dominant headache, which was used for the stratified analyses, was assigned according to the IHS criteria. The presence of concurrent types of headaches can, however, have influenced the interpretation of the findings; especially for the patients with a headache attributed to a TMD the majority also reported concurrent symptoms of migraine and/or TTH. Perhaps, the limited improvement over time in this group of patients is related to the higher complexity of complaints and should be studied further.  
The medication use of the patients was not tracked during the study, which could have provided more information on the perception and intensity of the complaints. Furthermore, to facilitate enough change over time in this mostly chronic patient population, an observational study design was chosen in which TMD-pain patients with headache were followed during a 12-week of usual care treatment. We did not intend to study the effects of a specific TMD-treatment and therefore no control group was included. In fact, the variation within the TMD-pain groups with different types of headaches allowed for the contrast needed in the regression analyses. The literature supports that attention, good communication and empathy improve the effects of all kinds of therapies, generating a placebo effect [37; 38]. In this study, the amount of attention provided by multiple staff persons in a specialty clinic has probably caused a placebo effect that was not measure because we believe that it is present in all subgroups and both complaints, so this would not influence on answering our research question.  
As this is a prospective observational study, there is always the risk of some bias [39]. Response bias could happen due to differences in the responses according to the patient's mood, for example. Using recommended and validated questionnaires should reduce the risk of response bias. Nevertheless, the risk of response bias was probably the same between the subgroups and did not influence the results of this study.  
For future studies, adding a control group which does not receive treatment or incorporate a pre-treatment period in the longitudinal design and extend the follow-up period, could make the conclusions more robust. Including daily diaries for headache, reporting the medication intake and including outcomes related to psychological aspects could provide additional information and is also encouraged.