One of the decisions facing every ParEvo exercise designer is how many iterations to include.
Eight iterations has become a common default, but there is no established optimum. Different numbers of iterations create different opportunities for storyline development and have different consequences for the survival of earlier alternatives.
Each new iteration allows participants to extend existing storylines, potentially introducing new developments. At the same time, storylines that receive no contributions become extinct.
As an exercise proceeds, more of the original storylines may disappear. Eventually, all the surviving storylines could descend from just one of the contributions made in the first iteration.
Importantly, this can happen even if participants select storylines entirely at random.
Exploring the consequences of different exercise designs
The ParEvo Iteration Planner allows you to explore what would happen under a simple model of random selection.
You can vary three parameters:
- Contributions per iteration: The number of participants making a contribution in each iteration, assumed to remain constant throughout the exercise.
- Number of iterations: The total number of iterations planned.
- Number of surviving lineages: A number chosen by you to investigate the probability that fewer than this number of original lineages will survive.
For example, selecting 3 for the third input calculates the probability that only one or two original lineages will remain after the specified number of iterations. It does not calculate the probability of exactly three surviving.
The application reports three results:
- The probability that all final storylines descend from just one original lineage.
- The probability that fewer than your chosen number of original lineages survive.
- The expected number of surviving original lineages.
The accompanying graph shows how the first two probabilities change as the number of iterations increases. Each point on a curve represents the probability of the specified outcome after the number of iterations shown on the horizontal axis.
You can explore different combinations of inputs to see how the results change.
The calculations are intended to inform your judgment, not to identify an optimal exercise design.
The trade-off between earlier and later storyline development
Increasing the number of iterations creates more opportunities to develop later parts of the narratives. This can allow new possibilities to emerge and existing possibilities to be explored in greater depth.
However, additional iterations also increase the likelihood that some of the original alternatives will disappear.
Consequently, longer exercises may produce final storylines with increasingly shared beginnings, while offering more opportunities for varied developments in their later stages.
Shorter exercises may preserve more of the original alternatives, but provide fewer opportunities to explore their subsequent consequences.
Neither outcome is necessarily preferable. The appropriate balance depends on the purpose of the exercise and the interests of its participants.
Why use random selection as a reference point?
The model draws on an idea from evolutionary biology known as genetic drift.
In a finite population, some genetic lineages can become dominant while others disappear simply through random reproduction, without any differences in their relative fitness.
A similar process can occur in ParEvo when participants select storylines for further development.
The model assumes that every participant independently selects one storyline at random from those available in the preceding iteration. Every available storyline has an equal probability of being selected, and multiple participants may select the same storyline.
In actual exercises, participants are unlikely to make entirely random choices. They may favour particular storylines for many different reasons. The model therefore provides a neutral reference point, not a prediction of actual behaviour.
It also measures only structural convergence. It does not measure the novelty, diversity, quality or usefulness of the narrative content.
The Iteration Planner is intended to make these structural consequences visible, helping facilitators make their own decisions about exercise design.
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