### X: regression value

It is possible to have in a data set one or several columns with column-type X. Within a given subject-occasion, string “.” will be interpolated (nearest neighbor interpolation is used) for **dose-lines only** (N.B.: if there is an EVID column dose-lines correspond to EVID = 1 or EVID = 4). Else wise, for measurement line, no interpolation is performed. If no regressor is defined on such a line, it will be replaced by a NaN. Therefore, in the following data set example,

ID TIME X AMT Y EVID Tom 0 . 1 . 1 Tom 5 1 . 12 0 Tom 10 . . 10 0 Tom 15 12 1.5 . 1 Tom 20 -6 . 8 0 Tom 25 . 0.2 . 4 Tom 30 . . 0.1 0

The evolution of X with respect to time is defined by the following figure.

Thus, X is set to

- X(0) = 1 (it is a dose-line so an interpolation is realized. The nearest interpolation is realized and here nearest sample corresponds to a response-line).
- X(5) = 1 (from direct reading of input file even if the line does not correspond to a dose).
- X(10) = NaN (regression is undefined in the input file but since it is not a dose-line, no interpolation is realized).
- X(15) = 2 (from direct reading of input file).
- X(20) = 3 (from direct reading of input file even though the line does not correspond to a dose).
- X(25) = 3 (it is a dose-line so an interpolation is realized. The nearest interpolation is realized and here nearest sample corresponds to a response-line).
- X(30) = NaN (regression is undefined in the input file but since it is not a dose-line, no interpolation is realized).

To add a valid information between time 10 and 15, for example X = 1.5, the data set should contain both a regressor value at time 10 along with the measurement value,

Tom 10 1.5 . 10 0

*Format restrictions (an exception will be thrown otherwise):*

- The regression-columns (i.e. columns with column-type X) shall contain either strings that can be converted to double or “.”.
- Each subject-occasion must contain at least one non “.” value (since it is then impossible to interpolate values).
- When there are several lines with the same time, the value of the regressor column must be the same.