A new paradigm established for the investigation of errors in frequency-domain measurements was applied to the phase sensitive detection (PSD) algorithm used in lock-in amplifiers, one of two techniques commonly used for spectroscopy measurements. For additive errors in time-domain signals, the errors in the phase angle and magnitude were found to be uncorrelated, even when time-domain errors were not normally distributed. In contrast to results obtained using the frequency response analysis algorithm, errors in the real and imaginary impedance were found to be correlated, and the variances of the real and imaginary parts of the complex impedance were not equal, except at a phase angle of -pi/4. A propagation of error model was established to explain and confirm the calculated statistical properties. The statistical characteristics of the results were in good agreement with experimental observations. (C) 2003 The Electrochemical Society.