Final answer:
The scenario describes a controlled experiment, not an observational study, since it involves the manipulation of variables by using aspirin and a placebo.
Step-by-step explanation:
If researchers are investigating whether taking aspirin regularly reduces the risk of a heart attack by dividing participants into two groups, where one group receives aspirin and the other receives a placebo without knowing which they are taking, this describes a controlled experiment. In contrast, an observational study would involve measuring characteristics in a sample without attempting to manipulate or control the variables of interest. An example of this could be a political poll where participants are asked about their ages and candidate preferences, providing a snapshot of opinions that may be associated with demographics.
It's important to distinguish between experiments and observational studies because the methodologies have different implications. While observational studies can indicate correlations, they cannot definitively establish causation. Experiments, on the other hand, can more reliably test hypotheses about causal relationships by introducing controls and manipulating variables. In the scenario provided, since there is a deliberate manipulation of variables with the use of aspirin and placebo, it falls under the definition of an experiment.