Final answer:
Observational studies cannot establish a causal link as they do not control for all possible variables and only observe relationships between variables. A cross-sectional study, such as one on soda consumption and diabetes, can reveal correlation but not causation.
Step-by-step explanation:
Observational studies, such as the one mentioned where daily sitting habits were measured before the assessment of obesity status, cannot definitively establish a causal link. This is primarily because there could be other variables affecting the outcome that aren't controlled for, which is a common issue with non-experimental research designs. In contrast to experimental studies, an observational study does not manipulate the independent variable but rather observes the relationship between variables.
For example, in a study investigating soda consumption and the development of diabetes, we cannot definitively say that soda consumption causes diabetes. This type of study is an example of a cross-sectional study, which collects data at one point in time. Even if a correlation between soda consumption and diabetes is found, there could be unaccounted variables, such as genetic predisposition or lifestyle factors, that influence the development of diabetes.
It is essential to distinguish between correlation and causation in these studies. Correlation indicates an association between two variables, but it does not imply that one causes the other. Hence, while observational studies can suggest potential links between variables, they cannot confirm causation without further experimental evidence.