Final answer:
Investigative techniques such as invigilation, surveillance, and covert operations are part of the personal observation approach, often used in law enforcement and intelligence gathering.
Step-by-step explanation:
The investigative techniques that include invigilation, surveillance, and covert operations fall under the approach of personal observation. These methods are part of the data collection process in which an observer monitors the activities or behaviors of individuals without their knowledge. This often requires the observer to be in close proximity to the subject without being detected, allowing for the collection of information in a natural setting. Personal observation is crucial in fields like law enforcement and intelligence gathering, as it can provide detailed and real-time evidence.