Several design issues may have undermined the existing study results, including small study samples, lack of long-term follow-up, incomplete follow-up, and lack of control groups (e.g. OA patients).
10 Some existing studies use sampling weights (Maclean 2014, 2015), but others do not (Kahn 2010; Hershbein 2012; Speer 2015).
Existing studies include samples with wide-ranging comorbidities, so we assessed relationships between severity of OSA and common sleep and psychological disturbances in recently diagnosed OSA patients with minimal co-morbidities.
The majority of existing studies have sampled adults, such as college students and corporate employees, whereas those that featured adolescents and children looked at those in grade school and beyond.
All volunteers were recruited within the target group of young families with children, using a snowball sampling approach, where existing study subjects recruit future subjects from among their acquaintances [37].
This approach collects a sample referred or nominated by the existing study participants, hence the term 'chain-referral method' was ascribed to this approach.
Oral swab samples and DBS were collected from dengue patients enrolled in existing clinical studies and from healthy volunteers enrolled in an existing serosurveillance study, from which venous blood was also collected within existing study protocols.
Many of the existing studies drew their samples from a single institution, had samples smaller than 500 individuals, or applied only bivariate or descriptive statistics [ 15].
In the few existing studies the clinical samples were small.
Furthermore, many of the existing studies have a small sample size and low statistical power to analyze associated factors.
