Practice Makes Permanent: Lock-In When Assignment Creates Its Own Evidence

Assigning each player to the position they already play best gives them more practice in that position, making the same lineup stronger tomorrow. In a five-player team game, each match consists of five players occupying the same five positions. Any lineup must stay close to the strongest combination that can currently be assembled. We prove an exact threshold on the tolerated decline in performance: below this threshold, always fielding the strongest lineup eventually makes every alternative player–position pair permanently unreachable; for five players starting at skill 0.40 on a scale where mastery is 1, the threshold is a 24% decline. When the tolerated decline is small enough, the lineup fielded in the first match becomes permanent. If skills are observed only through noisy tryouts, no rule does better than assigning a contested position to the player with the higher average tryout score. If uncertainty is too large, decision-makers who require proof that a new lineup is safe before trying it will not risk letting a player practice a new position. We tested teams of five to fifty players and found that the observed thresholds lie within 1.4% of our predictions; even if we balance how often each player gets a desired position, five of the ten desired player–position pairs, two for each player, remain permanently unreachable. Both performative prediction and safe exploration study this assignment–practice feedback loop; here, the thresholds are exact.