Non-approximability of precedence-constrained sequencing to minimize setups

It is a basic scheduling problem to sequence a set of precedence-constrained tasks to minimize the number of setups, where the tasks are partitioned into classes that require the same setup. We prove a conjecture in (Ph.D. Thesis, School of ISyE, Georgia Institute of Technology, August 1986; Oper. Res. 39 (1991) 1012) that no polynomial-time algorithm for this problem has constant worst-case performance ratio unless P=NP. A very simple algorithm has performance ratio $\text{n}$.