To understand and explain sample-based impression formation, it is necessary to consider both the Brunswikian uncertainty caused by sampling from the stimulus environment and the Thurstonian uncertainty arising from the cognitive processing thereof. Impression judgments must be formed in view of both sources of uncertainty. Even when an ecological sample of a target’s traits is held constant, the resulting distribution of target information in the judge’s mind can vary substantively as a function of semantic and affective responses, top-down inferences, and contextual influences. In the research reviewed in the present chapter, we investigate the interplay of Brunswikian and Thurstonian sampling in a person-impression task based on self-truncated trait samples, in which judges can stop sampling at the very moment when their internal mindset optimally prepares them to form a distinct impression. This task setting produces distinct self-truncation effects; the resulting impression judgments are polarized, conflict-free, and typically driven by small samples (after early truncation). When exactly the same traits are presented in a yoked-control design to other judges, who cannot exploit self-truncation effects, their judgment patterns are similar but clearly less pronounced, reflecting the same Brunswikian trait samples detached from the Thurstonian mindset.