Summary: The authors analyze the quadrature amplitude modulation (QAM) version of multilevel modulation with multistage decoding using a suboptimal metric, when transmission takes place over a memoryless Gaussian channel. The upper bounds for decoding error probabilities are functions of the Chernoff bounding parameter \(Z\). They argue that the conventional approximation of \(Z\) is not adequate; new values of \(Z\) that tightens the error bounds without causing them to lose their validity are given. The capacity for this system is also calculated, and they conclude that the use of a suboptimal metric in multistage decoding causes very little degradation in capacity compared to when the optimal metric is used in each decoding stage.