Step 1: Group by function:
Three of the four items name concrete data-gathering activities (asking questions directly, watching behaviour, and administering questionnaires over a population).
Step 2: Test the remaining item:
Objectivity does not describe an activity of gathering data at all; it describes an attitude/standard that should govern how any of those techniques is carried out.
Final Answer:
The technique missing from the data-collection list is 'Objectivity.'