Hello!

I'm running a logistic regression and I want to check for linearity, but I'm a little unsure how to do a box-tidwell transformation. I understand it's the crossproduct of each independent times it's natural log, and I've computed it to a new variable, but I don't know how to do this test in SPSS.

I've added box-tidwell transformed variables to a modell, and they are in Step 0, and significant, but they are not in the Equation in Step1

my syntax is:

COMPUTE t12.box.tidwell=t12 * LN(t12).

EXECUTE.

COMPUTE t28.box.tidwell=t28 * LN(t28).

EXECUTE.

COMPUTE t29.box.tidwell=t29 * LN(t29).

EXECUTE.

LOGISTIC REGRESSION VARIABLES isk.kat.0

/METHOD=ENTER t12 t28 t29 t12.box.tidwell t28.box.tidwell t29.box.tidwell

/CONTRAST (t12)=Indicator(1)

/CONTRAST (t29)=Indicator(1)

/CONTRAST (t28)=Indicator(1)

/CLASSPLOT

/PRINT=GOODFIT CORR CI(95)

/CRITERIA=PIN(0.05) POUT(0.10) ITERATE(20) CUT(0.5).

Would You so kind to help me solving this problem.

Thank You for your help!

Tarzicius