This work investigates the robustness of optimal decision rules under a changing size of a decision group where the competence of one member is outstanding while the others possess an identical competence. More specifically, we focus on the expert rule, the almost expert rule and the tie-breaking chairperson rule. We first establish criteria for the optimality of each of these rules and then use it to investigate their robustness. Robustness is an important property of decision rules, especially when cost of adjustment to a new rule is high. Our findings are that the expert rule and the chairperson rule are robust while the almost expert rule is not.