Final answer:
Cross-sectional studies are the most difficult to interpret among observational study designs because they only provide a snapshot at one point in time and lack the ability to establish a cause-and-effect relationship.
Step-by-step explanation:
Compared to other study designs, studies that use the cross-sectional design yield results that are the most difficult to interpret. This is because cross-sectional studies tend to be relatively cheap and easy to conduct but provide weaker results compared to other observational studies like case-control or cohort studies. These studies are often just a first step in research, used to identify variables that may be linked, which then may be investigated further in more rigorous study designs like case-control or cohort studies.
While case-control studies involve a retrospective comparison and cohort studies follow subjects over time, both providing more conclusive evidence, cross-sectional studies only provide a snapshot at one point in time, without information on the sequence of events or progression. Hence, it's more challenging to establish cause-and-effect relationships from a cross-sectional study.