How many scenarios are needed for a scenario exercise? The consensus
seems to be settling on either three or four scenarios. Here are the
In nearly every case, a single scenario is insufficient. A single
scenario can stimulate thought, but by itself it cannot support an
exploration of different options.
So, at least two scenarios are needed. This may be useful if it delimits
a range. However, since the brackets of the range are normally extreme,
they do not indicate how moderate action might achieve goals.
This brings us to three scenarios at a minimum. The problem with
three scenarios is that the middle scenario is likely to be
interpreted as a most likely or preferred scenario
by the users of the scenario. However, it is rarely the case that the
central scenario is thought to be most likely or preferred by its designers.
When possible, four scenarios is perhaps the best choice. In this
case there is no middle scenario and the users of the scenarios are more likely
to look at the full range of scenarios, rather than focusing on just one, as
they are likely to do in a three-scenario study. However, the more scenarios
there are, the more resources required to complete them. Also, more
attention is required from the audience, and
it is important to show respect for your audience by
not overwhelming them with a large volume of work.
More than four scenarios
The need to not overwhelm the audience rules out the possibility of using
more than four scenarios. It has been tried, and is generally thought to be
counter-productive. The one situation where this might not be the case is
when a small number of driving forces are combined in different combinations
to produce a large number of resulting scenarios. In this case, the results can
sometimes be presented in a grid or other arrangement that allows the patterns
to be grasped quickly.