I have 1 categorical variable called group (1 = treatment, 2 = placebo). If I create the survival curves with the Kaplan Meier method in SPSS I see that the survival is better for the treatment group compared to the placebo group. The survival curves are also significantly different according to the log-rank test.
When I do cox regression with the same variable (without adding other variables) the hazard ratio is significant as well and has a value below 1 (comparing treatment group to placebo group), indicating that the treatment group is associated with decreased hazard. So this is all as expected.
I can also create survival curves during the cox regression in SPSS. I can then seperate the lines for the group variable. However, these survival curves look different than the survival curves of the Kaplan Meier. How is this possible? Also the amount of vertical drops in the curve (which indicates that a patient has died) is also different between the cox regression and kaplan meier survival curves...The amount of vertical drops are somehow similar for the survival curves of the treatment group and placebo group in the cox regression...
Is the interpretation of the survival curves different for the cox regression than for the Kaplan Meier method? Is a vertical drop not a patient that died in cox regression? And how are the survival curves different for the cox regression and kaplan meier when I have the same variable and are comparing the same groups? Is this because the survival probability is calculated in another way?
I hope you can help we further and explain it in an easy way.
Thanks a lot for your help in advance!