8 This is because existing observational studies of the association between cord pH and outcomes have drawn inconsistent inferences, partly as a result of the different thresholds used to define abnormality, the variety of outcomes evaluated, and the different variables measured (arterial cord pH, venous cord pH, or base excess).
Because observational studies are the main source of information on QOF, we can only report on studies with rather weak evidence.
The decline in verbal learning across all exposure conditions was intriguing because of its consistency with observational studies of chronic exposure.
Although previously demonstrated in animal models (see references in [ 15]) the concept of heterotypic and heterosubtypic temporary immunity is very difficult to demonstrate in observational studies of humans, because it is rare to find sequential or contemporaneous circulation of different influenza types or sub-types.
We believe that this situation is very common in many observational studies of health because so many factors with transient effects are likely to influence the respondents' scores on both the health measure (e.g. depression) and the predictor (e.g. relationship problems).
Because this was an observational study of a convenience sample where patient charts were not sampled randomly, the generalizability of these study results to the entire European population may be limited.
The requirement for informed consent could be waived because this was an observational study of a standard diagnostic protocol, and all patients received the same type of diagnostics and care.
However, because this was an observational study of changes over time (conducted within a larger trial), we cannot conclusively state that the intervention activities themselves were more effective for the participants who were food insecure compared with those who were food secure.
The forced focus on one outcome measure in meta-analysis is common in systematic reviews of observational studies because of the lack of data on other outcome measures [ 11].
Measurement error of some type often arises in observational studies because of the use of surrogate variables (e.g., overall job task), self-reported behaviors (e.g., frequency of pesticide use), or other inexact measurements (Kauppinen 1994).
