Frequently, you may find yourself with a task box in your mental model that does not belong to any of the other towers in a mental space. This task box becomes a tower of it’s own, with just the one task box in it. Although I’ve said that the height of the tower is not significant, you have a special situation when there is just one voice represented by that task box and tower. If only one voice makes up the tower, treat that tower as a hypothesis. “Only one person mentioned this, and theoretically other people will mention this if we interview another 20 people.” The tower is still a valid tower because you expect other voices to join the singleton, should you gather more data. If you really don’t think other people would mention this item if you interview another 20 people, then remove that task box and tower from the mental model, as it is too unique to be significant.
Remember that the model is not an absolutely complete picture of people’s behavior. The model is a sketch of their behavior, where the mental spaces are pretty reliably represented, but not all the tasks necessarily appear in the towers. As time goes on and you have a chance to collect more data, you will be able to add it to existing mental spaces and towers.