In this context, the development and use of clinical practice guidelines (CPGs) are instruments of encounter between policies, citizens and professionals but they create many methodological and operational difficulties.
An examination of these more recent studies reveals the continued existence of many of the methodological difficulties referred to in previous reviews [ 6, 7].
The two year results highlight many of the methodological difficulties of such an approach, including the inherent difficulty with historical comparators, and the major uncertainties around the interpretation of the current results.
However, these studies faced many methodological obstacles, including difficulties obtaining accurate measures of alcohol use patterns and problems controlling for confounding factors such as variations in the HIV strain, patient age, demographics, ethnicity, time since infection, comorbidities, medication side effects, and a host of environmental influences.
Many articles of that time addressed the methodological difficulties inherent in assessing outcomes of complex interventions in CAM[ 6], and only sparse evidence from research underscored the role of possible modulating factors on outcomes of individualized care, both in CAM and COM.
A way to overcome the ethical and methodological difficulties of using patients for as many as three examinations is to spread them out over several days, as in the studies of Ljungquist et al. [ 4] and Horneij et al. [ 13].
While some animals' embryos have simpler geometries, being relatively flat with spherical or ellipsoidal cell layers (like the early Drosophila fly embryo), many types of embryos have inherently spatially three-dimensional cell order, adding methodological difficulties with respect to specimen thickness and optical nontransparency.
Some methodological difficulties recur across much of the cognitive and neuroscience research on gamblers' decision making: many gambling tasks do not involve ecologically valid stimuli or genuine monetary rewards, and even when real money is available, it is questionable whether the sums used have similar subjective value to seasoned gamblers and nongamblers.
Of course, to conduct an ensemble test, important methodological difficulties must be confronted, including how to choose populations and species to analyze, and how many cases are necessary to provide an adequate test.
Moreover, it is well known that studies and trials involving children are affected by many methodological [ 1] and ethical concerns [ 17] as well as by economic barriers [ 13] resulting in difficulties to perform high-quality paediatric studies [ 5, 22], compliant with the existing guidelines and recommendations for high-quality paediatric studies [ 10, 14].
