X13

1. Log-level test

pre-specified means not to be tested

Model for logs

• Transformations (to be applied in the given order)
  • Fixed regression effects
  • Interpolation of missing values
  • Log transformation
  • Length of period adjustment: pre-specified or automatic, if td included in the initial model (pre-specified or remove option)
• ARIMA
  • Pre-specified or airline
  • Pre-specified mean correction
• Regression
  • Pre-specified variables
  • Trading days (remove option)
  • Easter (remove option)
  • Leap year (remove option, no automatic pre-adjustment)

Model for levels

• Transformations
  • Fixed regression effects
  • Interpolation of missing values
• ARIMA
  • Pre-specified or airline
  • Pre-specified mean correction
• Regression
  • Pre-specified variables
  • Trading days (remove option)
  • Easter (remove option)
  • Leap year (remove option)

Automatic pre-adjustment is not taken into account in the model in levels. When logs are chosen and automatic pre-adjustment is enabled, the model after log/level test is adapted: the length of period variable is no longer considered in the regression and is replaced by a pre-adjustment (which corresponds, in the case of a leap year correction, to a fixed regression effect of $\pm 3 \%$). If automatic pre-adjustment is disabled, the length-of-period variable remains in the regression).

2. Trading days test

The model chosen in (1) is compared with the same model with/without trading days effects.

In the case of a log transformation, a pre-adjustment or a length-of-period regression variable will be considered, following the pre-adjustment option, as discussed above.

The selection is based on an AIC test