margins and contrast after stcox

[Andrea Gobbo]

Dear Statalist,
I'm fairly new to Stata and a novice regarding statistics, please apologize if I used dataex or CODE wrong. Let me first introduce the focus of my question. I have some failure-time data, here is a sample of the two variables of interest.

* Example generated by -dataex-. For more info, type help dataex
clear
input double stricture_lenght float MB_vs_SIS_nn_psmatch
  3 0
  4 0
2.5 0
1.5 0
  3 0
  3 0
  3 0
  3 0
  3 0
  2 0
  4 0
  5 0
  2 0
  6 0
  4 0
  4 0
  3 0
  2 0
  4 0
  4 0
  5 0
1.5 0
1.5 0
3.5 0
  5 0
  4 1
1.5 1
2.5 1
  2 1
  3 1
3.5 1
  3 1
  3 1
  6 1
  3 1
  4 1
  5 1
  5 1
  4 1
  3 1
  3 1
  3 1
  4 1
  3 1
  3 1
  3 1
  2 1
  4 1
2.5 1
1.5 1
  . .
  1 .
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1.5 .
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  1 .
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  1 .
  1 .
  2 .
  . .
  1 .
2.5 .
1.5 .
  1 .
  1 .
3.5 .
  1 .
  . .
  2 .
  . .
1.5 .
2.5 .
1.5 .
 .5 .
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  1 .
  2 .
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  4 .
1.5 .
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  3 .
  1 .
3.5 .
  1 .
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  . .
end

First, I set up my data with stset. Then, I ran the Cox regression.

 stcox c.stricture_lenght i.MB_vs_SIS_nn_psmatch Uretrotomie FLUSSOPOST , vce(cluster clust_var)

         Failure _d: failure_bef_156m
   Analysis time _t: FU_kaplan_plus12
  Exit on or before: time 156

Iteration 0:   log pseudolikelihood = -44.997185
Iteration 1:   log pseudolikelihood = -34.312221
Iteration 2:   log pseudolikelihood =  -32.92996
Iteration 3:   log pseudolikelihood = -32.878874
Iteration 4:   log pseudolikelihood = -32.878746
Refining estimates:
Iteration 0:   log pseudolikelihood = -32.878746

Cox regression with Breslow method for ties

No. of subjects =         50                            Number of obs =     50
No. of failures =         12
Time at risk    = 5,345.7488
                                                        Wald chi2(4)  =  53.12
Log pseudolikelihood = -32.878746                       Prob > chi2   = 0.0000

                                       (Std. err. adjusted for 25 clusters in clust_var)
----------------------------------------------------------------------------------------
                       |               Robust
                    _t | Haz. ratio   std. err.      z    P>|z|     [95% conf. interval]
-----------------------+----------------------------------------------------------------
      stricture_lenght |   1.877969   .4056031     2.92   0.004     1.229833     2.86768
1.MB_vs_SIS_nn_psmatch |   4.404797   2.674084     2.44   0.015     1.340215    14.47696
           Uretrotomie |   11.01003   13.12806     2.01   0.044      1.06375    113.9561
            FLUSSOPOST |   .8636839   .0208727    -6.06   0.000     .8237279    .9055779
----------------------------------------------------------------------------------------

The vce cluster is because I used a propensity score matching and adjusted the standard errors computing.

Now the point of the question. I would like to test level 1 versus level 0 of MB_vs_SIS_nn_psmatch (dichotomous) at different values of stricture_lenght (continuous; values: 1, 2, 3, 4, and 5). To do so, I was thinking about using margins and contrast commands. I have tried several times without success. I don't have all the attempts, but I will show you the last one (codes suggested by chatGBT):

 margins MB_vs_SIS_nn_psmatch, at(stricture_lenght=(1(1)5)) predict(hr) post

Predictive margins                                          Number of obs = 50
Model VCE: Robust

Expression: Predicted hazard ratio, predict(hr)
1._at: stricture_lenght = 1
2._at: stricture_lenght = 2
3._at: stricture_lenght = 3
4._at: stricture_lenght = 4
5._at: stricture_lenght = 5

------------------------------------------------------------------------------------------
                         |            Delta-method
                         |     Margin   std. err.      z    P>|z|     [95% conf. interval]
-------------------------+----------------------------------------------------------------
_at#MB_vs_SIS_nn_psmatch |
                    1 0  |   .7837271   .8449675     0.93   0.354    -.8723787    2.439833
                    1 1  |   3.452159   4.061674     0.85   0.395    -4.508576    11.41289
                    2 0  |   1.471815   1.645008     0.89   0.371     -1.75234    4.695971
                    2 1  |   6.483048   7.726402     0.84   0.401    -8.660423    21.62652
                    3 0  |   2.764023   3.304474     0.84   0.403    -3.712626    9.240673
                    3 1  |   12.17496   15.15627     0.80   0.422    -17.53077     41.8807
                    4 0  |   5.190751   6.773259     0.77   0.443    -8.084593    18.46609
                    4 1  |    22.8642   30.43905     0.75   0.453    -36.79524    82.52365
                    5 0  |    9.74807   14.02292     0.70   0.487    -17.73635    37.23249
                    5 1  |   42.93827   62.04979     0.69   0.489    -78.67707    164.5536
------------------------------------------------------------------------------------------

.  contrast rb0.MB_vs_SIS_nn_psmatch#r.stricture_lenght
variable stricture_lenght not found
r(111);

I am sure I'm missing something in the codes, can someone help me find the right syntax? Also, if possible, with a brief explanation of the operators used for this specific case.
Thank you