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29.18.    Absence of residual seasonality. The most fundamental requirement of seasonal adjustment is that there be no estimable seasonal effect, known as residual seasonality, still present in the seasonally adjusted series. To detect residual seasonality and residual trading-day effects, validation should be performed using spectral diagnostics as well as other tools included in the seasonal adjustment packages, perhaps complemented by graphical diagnostics and statistical tests from external statistical packages. Both TRAMO-SEATS and X-12-ARIMA provide a wide range of quality measures and diagnostics for this purpose. 

29.19.    Stability and lack of bias. Other important requirements for good seasonal adjustment are a lack of bias in the level of the series and stability of the estimates. A lack of bias implies that the level of the seasonally adjusted series is similar to the level of the original series. Stability of the estimates implies that the inclusion of new data into the estimation procedures will not result in large changes in the estimates. Large revisions can indicate that the estimates are misleading or even meaningless.