Error Vector Magnitude (EVM) is a system-level parameter that is specified for most advanced communication standards. EVM measurement often takes extensive test development efforts, tester resources, and long test times. Since EVM is analytically related to system impairments, which are typically measured in a production test environment, it can be eliminated from the test list if the relations between EVM and system impairments are derived in a manner that is independent of the circuit implementation and manufacturing process. In this paper, we focus on the WLAN standard, and derive the relations between EVM and three of the most detrimental impairments for QAM/OFDM based systems: IQ imbalance, non-linearity, and noise. Simulations and hardware experiments show that the accuracy of the analytical models is in par with a direct EVM measurement with a reasonable test time.