Final answer:
Descriptions can be based on both clinical impressions and empirical evidence, as the two can complement each other in scientific research and understanding.
Step-by-step explanation:
Descriptions can be based on clinical impressions, which are the informal observations and opinions of professionals based on their experience in the field. However, descriptions, especially in scientific research, are more reliably based on empirical evidence, which is the evidence that comes from direct observation or scientifically gathered data. Descriptions are often a combination of both, as clinical impressions can guide what areas to study empirically. Therefore, when answering the question 'What are descriptions based on?', the correct option is 3) Both clinical impressions and empirical evidence.