The effective distance (d/sub eff/) of error events for partial response maximum likelihood (PRML) systems in magnetic recording channels are calculated using a realistic model for medium noise. The presence of medium noise has a strong negative impact on the minimum effective distances. The effective distances and the hierarchy of error events depend on the characteristics of the noise. In addition, due to the data dependency of medium noise, each error event corresponds to a range of d/sub eff/ depending upon which data pattern was transmitted.