Empirical studies have found that the non-accelerating inflation rate of unemployment (NAIRU) has been fluctuating in OECD countries around a constant mean of several percentage points over the past decades. This mean is calculated from the constant terms carried over from the wage and price growth equations. In this paper it is shown that a high proportion of the constant term is a statistical artefact and suggests a new method which yields approximately unbiased estimates of the time-invariant NAIRU. Using data for OECD countries it is shown that the constant-term correction lowers the unadjusted time-invariant NAIRU by approximately half