Partition priming in judgments of imprecise probabilities

On grounds of insufficient reason, a probability of 1/K is assigned to K mutually exclusive possible events when nothing is known about the likelihood of those events. Fox and Rottenstreich (2003) present evidence that subjective probability judgments are typically biased towards this ignorance prior, and therefore depend on the partition K. Results from two studies indicate that lower-upper (im18 precise) probability judgments by näıve judges also exhibit partition dependence, despite the potential that imprecise probabilities provide for avoiding it. However, beta regression reveals two kinds of priming effects, one of which is modeled by mixture distributions. Another novel finding suggests that when partition primes conflict with a normatively correct partition some judges widen their probability intervals to encompass both partitions. The results indicate that imprecise probability judgments may be better suited than precise probabilities for handling conflicting or ambiguous information about partitions.