in my retrospective study I have continuous and categorical variables. These latter are divided in two or more sub-groups. I want to analyze their impact on overall survival. In particular, I’m interested in a dichotomous variable (which I’m gonna call myVariable) and I suspect that its impact may be affected by age and sex. First, I categorized the continuous variables in 2 or more sub-categories and I performed a univariate analysis by the log-rank test with kaplan-meyer survival plots; myVariable was not significant.
On multivariate analysis, I used the Cox regression controlling for age and sex: I put age and sex in the first block and the other variables in the second, using the enter method. I included myVariable in the second block, to verify if it became significant after controlling for age and sex. Although I had previously categorized age and other continuous variables, I now used the original uncategorized data, when possible.
1 Is this procedure correct?
2 On Cox regression myVariable became significant. Strangely, age was not significant on log-rank test but became significant on Cox regression. How comes?
3 I’m using SPSS. In the “variable view” I set variables as string or numeric as appropriate. However, each variable is set as “Nominal”. Is that important? In other words, can it affect the results?
Thank you for your advices. Andrea