When a sampled individual is observed over a pre-determined period of time in structured observation, it is called

Observing one sampled individual for a pre-set span of time in structured observation is called focus sampling.

Attention stays fixed on that single subject throughout the chosen period.

Observed sampling is not a standard technical term in structured observation.

Random sampling is a method of selecting units by chance, not a way of timing observation.

Purposive sampling deliberately chooses cases by relevance, again unrelated to the timing scheme.

Time sampling, by contrast, records behaviour only within fixed intervals rather than one continuous span.