Final answer:
An example of an observational study is analyzing existing medical records to study the prevalence of a disease. This type of study is common when it's unethical or impractical to conduct experimental research, and it allows for the observation of a large number of subjects.
Step-by-step explanation:
An example of an observational study is option d) Analyzing existing medical records to study the prevalence of a disease. In an observational study, researchers observe and measure characteristics without manipulating the environment or the participants. This approach is often employed when it's unethical or impractical to conduct experimental studies, such as deliberately exposing subjects to harmful conditions. For instance, it wouldn't be ethical to conduct an experiment on the effects of malnutrition on school performance by deliberately causing malnutrition in a group of children. Instead, researchers may observe children who are already experiencing malnutrition and record their school performance, comparing it to children who are not malnourished.
Analyzing existing medical records is a way researchers can retrospectively examine data to find patterns and relationships. This method is less expensive and can involve more subjects than an experimental study, but it can also be subject to observer bias. To minimize this, clear criteria for recording and classifying behaviors should be established, and inter-rater reliability should be assured by comparing observations by multiple observers.