This is simply observing nature exactly as it occurs. The observer must hope that the data obtained are relevant and that the observation itself did not affect the behavior. Often we cannot gather information on every conceivable event. This category is the explicit recognition that many things are ignored. It is the most restricted but has the advantage of speed and that the desired information will always be included.
Contrived observations alter the situation.
In a sense experiments could be seen as extremely contrived observations. If major changes are made which allow a comparison, then it is called an experiment. Obviously however, it is a name for a position on a continuum. Experimental techniques do whatever is necessary to reveal what you need to know to better understand the world. Building on the previous metaphor, if knowledge were food, then experimentation is getting a meal by making it yourself.