Germán Rodríguez
Generalized Linear Models Princeton University

Methadone Treatment of Heroin Addicts

The survival time we will work with is the time that heroin addicts stay in a clinic for methadone maintenance treatment. The data were first analyzed by Caplehorn and Bell (1991) and appeared in a handbook of small datasets by Hand et al (1994). They have also been analyzed by Rabe-Hesketh and Everitt (2006) in various editions of their Handbook of Statistical Analyses using Stata. The data are available in the datasets section in a Stata file called heroin.dta.

The data come from two clinics coded 1 and 2; status is 1 if the event occurred (the person left the clinic) and 0 otherwise, and time is in days. The other two variables are an indicator of whether the person had a prison record, and the dose of methadone in mg.

[1] Models with No Covariates

(a) Verify that we have 150 failures in a total of 95,812 days of exposure. It will be convenient to recon time in months, treating a month as 30 days. Assume the hazard is constant over time. What's the event rate per month? What's the probability that someone would still be in treatment after one, two and three years?

. use http://data.princeton.edu/wws509/datasets/heroin, clear
(Retention of Heroin Addicts in Methadone Treatment Program, Hand(1994))

. count if status==1
  150

. scalar  count = r(N)

. quietly sum time

. di r(sum)
95812

. gen months = time / 30

. stset months, fail(status == 1) id(id)

                id:  id
     failure event:  status == 1
obs. time interval:  (months[_n-1], months]
 exit on or before:  failure

──────────────────────────────────────────────────────────────────────────────
        238  total observations
          0  exclusions
──────────────────────────────────────────────────────────────────────────────
        238  observations remaining, representing
        238  subjects
        150  failures in single-failure-per-subject data
   3193.733  total analysis time at risk and under observation
                                                at risk from t =         0
                                     earliest observed entry t =         0
                                          last observed exit t =  35.86666

. scalar hazard = count/r(sum)

. di hazard
.04696698

. di exp(-12*hazard), exp(-24*hazard), exp(-36*hazard)
.56915429 .3239366 .18436991
> library(foreign)

> heroin <- read.dta("http://data.princeton.edu/wws509/datasets/heroin.dta")

> library(dplyr)

> tally <- summarize(heroin, events=sum(status==1), days=sum(time),
+ 	exposure = days/30); tally
  events  days exposure
1    150 95812 3193.733

> hazard <- tally$events/tally$exposure; hazard
[1] 0.04696698

> exp(-hazard * c(12, 24, 36))
[1] 0.5691543 0.3239366 0.1843699

We have 150 failures in 95,812 days as expected. The monthly event rate is 0.047, so addicts leave treatment at the rate of 1 out of 20 per month. The probability of remaining in treatment is 57% after one year, 32% after two and 18% after three years.

(b) Split the dataset so that we have separate observations for 0-3, 3-6, 6-12, 12-18, 18-24 and 24-36 months. (In other words split the first year into two quarters and a semester, and the second year into two semesters.) There are no exits after 3 years. Fit a piece-wise exponential model and describe the shape of the hazard. Test the hypothesis that the hazard is constant. Estimate the probability that someone would be in treatment after one, two and three years.

. stsplit interval, at(3 6 12 18 24 36)
(613 observations (episodes) created)

. gen events = _d

. gen exposure = _t - _t0

. poisson events i.interval, exposure(exposure)

Iteration 0:   log likelihood = -512.97744  
Iteration 1:   log likelihood = -512.89225  
Iteration 2:   log likelihood = -512.89205  
Iteration 3:   log likelihood = -512.89205  

Poisson regression                              Number of obs     =        851
                                                LR chi2(5)        =      10.01
                                                Prob > chi2       =     0.0749
Log likelihood = -512.89205                     Pseudo R2         =     0.0097

─────────────┬────────────────────────────────────────────────────────────────
      events │      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
─────────────┼────────────────────────────────────────────────────────────────
    interval │
          3  │   .1847856    .291796     0.63   0.527    -.3871241    .7566953
          6  │   .2764494   .2616835     1.06   0.291    -.2364409    .7893396
         12  │   .3810534   .2752023     1.38   0.166    -.1583333      .92044
         18  │     .45605   .3198465     1.43   0.154    -.1708377    1.082938
         24  │   1.056881   .3318809     3.18   0.001     .4064063    1.707355
             │
       _cons │  -3.364343   .2085144   -16.13   0.000    -3.773024   -2.955662
ln(exposure) │          1  (exposure)
─────────────┴────────────────────────────────────────────────────────────────

. di e(chi2), e(df_m)
10.013372 5

. mat list e(b)

e(b)[1,7]
        events:     events:     events:     events:     events:     events:     events:
            0b.          3.          6.         12.         18.         24.            
      interval    interval    interval    interval    interval    interval       _cons
y1           0   .18478562   .27644938   .38105336   .45604996   1.0568809  -3.3643429

. mata b = st_matrix("e(b)")

. mata h = exp(b[7] :+ b[1..6])

. mata w = 3, 3, 6, 6, 6, 12

. mata S = exp(-runningsum(h :* w))

. mata S[(3, 5, 6)]
                 1             2             3
    ┌───────────────────────────────────────────┐
  1 │  .6052257103   .3219616817     .09754079  │
    └───────────────────────────────────────────┘
> library(survival)

> heroin <- mutate(heroin, months = time/30, fail = status) # for clarity

> breaks <- c(3, 6, 12, 18, 24, 36)

> heroinx <- survSplit(heroin, cut = breaks, start = "start", end = "months",
+ 	event = "fail", episode = "interval")	

> heroinx <- mutate(heroinx, exposure = months - start, interval = 
+ 	factor(interval, labels=paste("(",c(0,breaks[-6]),",",breaks,"]", sep = "")))

> nrow(heroinx)
[1] 851

>  	pwe <- glm(fail ~ interval + offset(log(exposure)), data = heroinx, family = poisson)

> summary(pwe)$coefficients
                  Estimate Std. Error     z value     Pr(>|z|)
(Intercept)     -3.3643429  0.2085120 -16.1350089 1.447867e-58
interval(3,6]    0.1847856  0.2917936   0.6332751 5.265540e-01
interval(6,12]   0.2764494  0.2616729   1.0564692 2.907539e-01
interval(12,18]  0.3810534  0.2751989   1.3846471 1.661604e-01
interval(18,24]  0.4560500  0.3198416   1.4258618 1.539082e-01
interval(24,36]  1.0568809  0.3318793   3.1845343 1.449871e-03

> me <- glm(fail ~ offset(log(exposure)), data=heroinx, family=poisson); exp(coef(me))
(Intercept) 
 0.04696698 

> deviance(me) - deviance(pwe)
[1] 10.01337

> b <- coef(pwe)

> h <- exp(b[1] + c(0,b[-1]))

> w <- c(3, 3, 6, 6, 6, 12)

> exp(-cumsum(h * w))
                  interval(3,6]  interval(6,12] interval(12,18] interval(18,24] 
     0.90144685      0.79567346      0.60522571      0.44668129      0.32196168 
interval(24,36] 
     0.09754079 

The rate at which addicts leave the clinic increases substantially and steadily with duratio of stay, at the one year mark it is almost 50% higher than in the first trimester, and in the third year it is almost three times what it was initially. To test the hypothesis of a constant hazard we compare this model with the null, which gives as a chi-squared of 10.01 on 5 d.f., a test that would accept at the conventional 5% level. The estimated survival probabilities are 61% after one year, 32% after two and just under 10% after three years.

[2] Clinic Effects

(a) Introduce a dummy variable to identify clinic 1 and add it to the model. Interpret the exponentiated coefficient and test its significance. (Note that clinic is coded 1 and 2, we want to treat clinic 2 as the reference. You may want to save this model for part c.)

. gen clinic1 = clinic == 1

. poisson events i.interval clinic1, exposure(exposure)

Iteration 0:   log likelihood = -497.08351  
Iteration 1:   log likelihood = -497.03449  
Iteration 2:   log likelihood = -497.03443  
Iteration 3:   log likelihood = -497.03443  

Poisson regression                              Number of obs     =        851
                                                LR chi2(6)        =      41.73
                                                Prob > chi2       =     0.0000
Log likelihood = -497.03443                     Pseudo R2         =     0.0403

─────────────┬────────────────────────────────────────────────────────────────
      events │      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
─────────────┼────────────────────────────────────────────────────────────────
    interval │
          3  │   .1932776   .2917984     0.66   0.508    -.3786367    .7651919
          6  │   .3130266   .2617334     1.20   0.232    -.1999614    .8260146
         12  │   .4588443   .2754258     1.67   0.096    -.0809804     .998669
         18  │   .6027036   .3205791     1.88   0.060    -.0256198    1.231027
         24  │   1.336127   .3348078     3.99   0.000     .6799155    1.992338
             │
     clinic1 │   1.085154   .2121442     5.12   0.000      .669359    1.500949
       _cons │  -4.212952   .2776001   -15.18   0.000    -4.757038   -3.668866
ln(exposure) │          1  (exposure)
─────────────┴────────────────────────────────────────────────────────────────

. estimates store clinic

. di exp(_b[clinic1]) - 1
1.9598958
> heroinx <- mutate(heroinx, clinic1 = as.numeric(clinic == 1))

> pwec <-  update(pwe, . ~ . + clinic1)

> summary(pwec)$coefficient
                  Estimate Std. Error     z value     Pr(>|z|)
(Intercept)     -4.2129522  0.2775893 -15.1769264 5.027959e-52
interval(3,6]    0.1932776  0.2917943   0.6623762 5.077302e-01
interval(6,12]   0.3130266  0.2617218   1.1960284 2.316855e-01
interval(12,18]  0.4588443  0.2754218   1.6659693 9.571952e-02
interval(18,24]  0.6027036  0.3205751   1.8800700 6.009854e-02
interval(24,36]  1.3361267  0.3348046   3.9907653 6.586044e-05
clinic1          1.0851541  0.2121327   5.1154493 3.129949e-07

> exp(coef(pwec)["clinic1"]) - 1
 clinic1 
1.959896 

The departure rate is much higher in clinic 1 than clinic, in fact almost triple at the same duration of treatment, a highly significant effect with a Wald z-test of 5.12.

(b) Let us add an interaction between clinic and duration. To save d.f. we will group time in years for purposes of the interaction. For full credit present your parameter estimates in terms of the effect of clinic 1 compared to 2 in the first, second and third year of treatment. Comment on the point estimates.

To achieve this goal we create separate dummy variables to represent the contrast betweeen clinics 1 and 2 in each year and then fit a model.

. gen clinic1year1 = clinic1 * (interval < 12)

. gen clinic1year2 = clinic1 * (interval >= 12 & interval < 24)

. gen clinic1year3 = clinic1 * (interval >= 24)

. poisson events i.interval clinic1year*, exposure(exposure)

Iteration 0:   log likelihood = -494.51171  
Iteration 1:   log likelihood = -491.19934  
Iteration 2:   log likelihood = -491.15455  
Iteration 3:   log likelihood = -491.15449  
Iteration 4:   log likelihood = -491.15449  

Poisson regression                              Number of obs     =        851
                                                LR chi2(8)        =      53.49
                                                Prob > chi2       =     0.0000
Log likelihood = -491.15449                     Pseudo R2         =     0.0516

─────────────┬────────────────────────────────────────────────────────────────
      events │      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
─────────────┼────────────────────────────────────────────────────────────────
    interval │
          3  │   .1897187   .2918028     0.65   0.516    -.3822043    .7616417
          6  │   .2975722   .2618246     1.14   0.256    -.2155945     .810739
         12  │  -.3384617   .4922646    -0.69   0.492    -1.303283    .6263592
         18  │  -.1761591   .5079844    -0.35   0.729     -1.17179     .819472
         24  │  -.7497782   1.041067    -0.72   0.471    -2.790233    1.290676
             │
clinic1year1 │   .5491248   .2549685     2.15   0.031     .0493957    1.048854
clinic1year2 │   1.505471   .4095048     3.68   0.000     .7028567    2.308086
clinic1year3 │   3.080406   1.035098     2.98   0.003      1.05165    5.109161
       _cons │  -3.769108   .2895188   -13.02   0.000    -4.336554   -3.201661
ln(exposure) │          1  (exposure)
─────────────┴────────────────────────────────────────────────────────────────

. mata exp(st_matrix("e(b)")[7..9])
                 1             2             3
    ┌───────────────────────────────────────────┐
  1 │  1.731736713   4.506277465   21.76723125  │
    └───────────────────────────────────────────┘

. estimates store model2b
> heroinx <- mutate(heroinx, code = as.numeric(interval),
+ 	clinic1year1 = clinic1 * (code <= 3),
+ 	clinic1year2 = clinic1 * (code > 3 & code < 6),
+ 	clinic1year3 = clinic1 * (code == 6))

> pwei <-  update(pwe, . ~ . + clinic1year1 + clinic1year2 + clinic1year3)

> summary(pwei)$coefficient
                  Estimate Std. Error     z value     Pr(>|z|)
(Intercept)     -3.7691076  0.2895117 -13.0188447 9.560644e-39
interval(3,6]    0.1897187  0.2918003   0.6501662 5.155848e-01
interval(6,12]   0.2975722  0.2618153   1.1365732 2.557167e-01
interval(12,18] -0.3384617  0.4922541  -0.6875751 4.917204e-01
interval(18,24] -0.1761591  0.5079744  -0.3467875 7.287510e-01
interval(24,36] -0.7497782  1.0410633  -0.7202042 4.713993e-01
clinic1year1     0.5491248  0.2549585   2.1537813 3.125733e-02
clinic1year2     1.5054714  0.4094973   3.6763887 2.365590e-04
clinic1year3     3.0804057  1.0350963   2.9759606 2.920724e-03

> exp(coef(pwei)[7:9])
clinic1year1 clinic1year2 clinic1year3 
    1.731737     4.506277    21.767232 

We see that the difference between the clinics increases substantially with duration. In the first year the exit rate in clinic 2 is 73% higher than in clinic 1, but in year 2 it is more than four times as high, and in year 3 it is a remarkable 22 times as high. (Turns out clinic 1 pretty much restricts the duration of stay to two years.)

  1. Test the hypothesis that the clinic effect does indeed vary by year using a likelihood ratio test and a Wald test.
. lrtest clinic .

Likelihood-ratio test                                 LR chi2(2)  =     11.76
(Assumption: clinic nested in model2b)                Prob > chi2 =    0.0028

. test clinic1year1 == clinic1year2 == clinic1year3

 ( 1)  [events]clinic1year1 - [events]clinic1year2 = 0
 ( 2)  [events]clinic1year1 - [events]clinic1year3 = 0

           chi2(  2) =    8.51
         Prob > chi2 =    0.0142
> deviance(pwec) - deviance(pwei)
[1] 11.75989

> A <- matrix( c(-1, 1, 0, -1, 0, 1), 2, 3, byrow = TRUE)

> b <- A %*% coef(pwei)[7:9]

> V <- A %*% vcov(pwei)[7:9, 7:9] %*% t(A)

> t(b) %*% solve(V) %*% b
         [,1]
[1,] 8.514599

For the likelihood ratio test we compare the models with and without the interaction, obtaining a chi-squared of 11.76 on 2 d.f., which is significant at the 1% level. For the Wald test we simply test if the clinic effects to be the same each year, obtaining 8.51 on the same d.f., with a P-value of 0.014. (I thought it would be instructive to do the calculation from first principles.)

[3] Prison and Methadone

(a) Starting from the model of part 2.b where the effect of clinic varies by year, add a dummy variable for prison history and interpret the resulting estimate.

. poisson events i.interval clinic1year* prison, exposure(exposure)

Iteration 0:   log likelihood = -492.67668  
Iteration 1:   log likelihood = -489.51457  
Iteration 2:   log likelihood = -489.47392  
Iteration 3:   log likelihood = -489.47387  

Poisson regression                              Number of obs     =        851
                                                LR chi2(9)        =      56.85
                                                Prob > chi2       =     0.0000
Log likelihood = -489.47387                     Pseudo R2         =     0.0549

─────────────┬────────────────────────────────────────────────────────────────
      events │      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
─────────────┼────────────────────────────────────────────────────────────────
    interval │
          3  │   .1918866   .2918031     0.66   0.511     -.380037    .7638103
          6  │   .3192309   .2621104     1.22   0.223     -.194496    .8329578
         12  │  -.3353302   .4926043    -0.68   0.496    -1.300817    .6301564
         18  │  -.1773817   .5077096    -0.35   0.727    -1.172474    .8177109
         24  │  -.7688194    1.04115    -0.74   0.460    -2.809436    1.271797
             │
clinic1year1 │   .5555696    .255022     2.18   0.029     .0557356    1.055403
clinic1year2 │   1.551366   .4103486     3.78   0.000     .7470973    2.355634
clinic1year3 │   3.160547   1.036035     3.05   0.002     1.129956    5.191137
      prison │   .3048991   .1653632     1.84   0.065    -.0192068     .629005
       _cons │  -3.925787   .3036952   -12.93   0.000    -4.521018   -3.330555
ln(exposure) │          1  (exposure)
─────────────┴────────────────────────────────────────────────────────────────

. di exp(_b[prison]) - 1
.35648811
> pweip <- update(pwei, . ~ . + prison)

> summary(pweip)$coefficients
                  Estimate Std. Error     z value     Pr(>|z|)
(Intercept)     -3.9257867  0.3036874 -12.9270634 3.166853e-38
interval(3,6]    0.1918866  0.2917996   0.6575973 5.107969e-01
interval(6,12]   0.3192309  0.2621010   1.2179691 2.232357e-01
interval(12,18] -0.3353302  0.4925842  -0.6807572 4.960251e-01
interval(18,24] -0.1773817  0.5076902  -0.3493897 7.267967e-01
interval(24,36] -0.7688194  1.0411455  -0.7384361 4.602495e-01
clinic1year1     0.5555696  0.2550123   2.1785989 2.936148e-02
clinic1year2     1.5513658  0.4103303   3.7807734 1.563419e-04
clinic1year3     3.1605431  1.0360323   3.0506221 2.283678e-03
prison           0.3048991  0.1653586   1.8438663 6.520262e-02

> exp(coef(pweip)["prison"]) - 1
   prison 
0.3564881 

At any given duration of treatment in either clinic, the rate of departure is 36% higher for patients with a prison record than for those without.

(b) Add a linear effect of dose and interpret the estimate. The effect of 1 mg of methadone may not be of interest because doses vary by several mg. In fact the standard deviation is 14.45. What's the effect on the hazard of increasing the dose by one standard deviation?

. poisson events i.interval clinic1year* prison dose, exposure(exposure)

Iteration 0:   log likelihood = -477.56757  
Iteration 1:   log likelihood = -474.29046  
Iteration 2:   log likelihood = -474.24806  
Iteration 3:   log likelihood =   -474.248  
Iteration 4:   log likelihood =   -474.248  

Poisson regression                              Number of obs     =        851
                                                LR chi2(10)       =      87.30
                                                Prob > chi2       =     0.0000
Log likelihood =   -474.248                     Pseudo R2         =     0.0843

─────────────┬────────────────────────────────────────────────────────────────
      events │      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
─────────────┼────────────────────────────────────────────────────────────────
    interval │
          3  │   .2392676    .291938     0.82   0.412    -.3329203    .8114555
          6  │    .417069   .2626636     1.59   0.112    -.0977421    .9318802
         12  │  -.2169191   .4935739    -0.44   0.660    -1.184306     .750468
         18  │     .00888   .5075821     0.02   0.986    -.9859625    1.003723
         24  │  -.3725992     1.0441    -0.36   0.721    -2.418998      1.6738
             │
clinic1year1 │   .4582848   .2552573     1.80   0.073    -.0420102    .9585798
clinic1year2 │   1.502897    .411861     3.65   0.000     .6956643     2.31013
clinic1year3 │   2.997887   1.036895     2.89   0.004     .9656101    5.030165
      prison │    .361036   .1668066     2.16   0.030     .0341009     .687971
        dose │  -.0350766   .0063872    -5.49   0.000    -.0475952    -.022558
       _cons │  -1.859145     .45847    -4.06   0.000     -2.75773   -.9605601
ln(exposure) │          1  (exposure)
─────────────┴────────────────────────────────────────────────────────────────

. estimates store model3b

. di exp(14.45 * _b[dose]) - 1
-.39761399
> pweip <- update(pwei, . ~ . + prison)

> summary(pweip)$coefficients
                  Estimate Std. Error     z value     Pr(>|z|)
(Intercept)     -3.9257867  0.3036874 -12.9270634 3.166853e-38
interval(3,6]    0.1918866  0.2917996   0.6575973 5.107969e-01
interval(6,12]   0.3192309  0.2621010   1.2179691 2.232357e-01
interval(12,18] -0.3353302  0.4925842  -0.6807572 4.960251e-01
interval(18,24] -0.1773817  0.5076902  -0.3493897 7.267967e-01
interval(24,36] -0.7688194  1.0411455  -0.7384361 4.602495e-01
clinic1year1     0.5555696  0.2550123   2.1785989 2.936148e-02
clinic1year2     1.5513658  0.4103303   3.7807734 1.563419e-04
clinic1year3     3.1605431  1.0360323   3.0506221 2.283678e-03
prison           0.3048991  0.1653586   1.8438663 6.520262e-02

> exp(coef(pweip)["prison"]) - 1
   prison 
0.3564881 

The coefficient indicates that the hazard is 3.5% lower per mg of methadone. (The value is small enough that it can be interpreted directly as a percent.) A dose increase of one standard deviation or 14.45 mg is asociated with a 39.8% reduction in the risk of leaving treatment.

(c) The original paper by Caplehorn and Bell treated dose as a categorical variable with three levels: < 60, 60-79, and 80+. Compare this specification with the linear term in part b in terms of parsimony and goodness of fit.

. recode dose (min/59 = 1 "< 60") (60/79=2 "60-79") (80/max=3 "80+"), ///
>     gen(doseg)
(851 differences between dose and doseg)

. scalar aic1 = -2*e(ll) + 2*(e(df_m) + 1)

. poisson events i.interval clinic1year* prison i.doseg, exposure(exposure)

Iteration 0:   log likelihood = -477.36196  
Iteration 1:   log likelihood = -474.03756  
Iteration 2:   log likelihood = -473.99224  
Iteration 3:   log likelihood = -473.99218  
Iteration 4:   log likelihood = -473.99218  

Poisson regression                              Number of obs     =        851
                                                LR chi2(11)       =      87.81
                                                Prob > chi2       =     0.0000
Log likelihood = -473.99218                     Pseudo R2         =     0.0848

─────────────┬────────────────────────────────────────────────────────────────
      events │      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
─────────────┼────────────────────────────────────────────────────────────────
    interval │
          3  │   .2276247   .2918845     0.78   0.435    -.3444585    .7997079
          6  │   .3914805    .262388     1.49   0.136    -.1227904    .9057515
         12  │  -.1965312   .4931713    -0.40   0.690    -1.163129    .7700669
         18  │   .0461413   .5087045     0.09   0.928    -.9509013    1.043184
         24  │  -.2437024   1.046734    -0.23   0.816    -2.295263    1.807858
             │
clinic1year1 │   .4298679    .256712     1.67   0.094    -.0732784    .9330143
clinic1year2 │   1.432044   .4141453     3.46   0.001     .6203341    2.243754
clinic1year3 │   2.858089   1.042954     2.74   0.006     .8139374    4.902241
      prison │   .3278549   .1689952     1.94   0.052    -.0033695    .6590793
             │
       doseg │
      60-79  │  -.5739452   .1791532    -3.20   0.001    -.9250791   -.2228114
        80+  │  -1.471106   .3014925    -4.88   0.000    -2.062021   -.8801918
             │
       _cons │  -3.438688   .3095259   -11.11   0.000    -4.045348   -2.832029
ln(exposure) │          1  (exposure)
─────────────┴────────────────────────────────────────────────────────────────

. scalar aic2 = -2*e(ll) + 2*(e(df_m) + 1)

. di aic1, aic2
970.49601 971.98437
> heroinx <- mutate(heroinx, doseq = cut(dose, c(0, 60, 80, Inf), right = FALSE))

> pweipq <- update(pweip, . ~ . + doseq)

> summary(pweipq)$coefficients
                   Estimate Std. Error      z value     Pr(>|z|)
(Intercept)     -3.43868815  0.3095178 -11.10982435 1.123804e-28
interval(3,6]    0.22762470  0.2918812   0.77985399 4.354768e-01
interval(6,12]   0.39148053  0.2623803   1.49203491 1.356900e-01
interval(12,18] -0.19653116  0.4931548  -0.39851820 6.902482e-01
interval(18,24]  0.04614126  0.5086887   0.09070627 9.277260e-01
interval(24,36] -0.24370241  1.0467283  -0.23282300 8.158988e-01
clinic1year1     0.42986790  0.2567020   1.67457952 9.401676e-02
clinic1year2     1.43204394  0.4141315   3.45794498 5.443125e-04
clinic1year3     2.85808901  1.0429504   2.74038830 6.136664e-03
prison           0.32785488  0.1689909   1.94007465 5.237062e-02
doseq[60,80)    -0.57394526  0.1791488  -3.20373422 1.356576e-03
doseq[80,Inf)   -1.47110621  0.3014871  -4.87949950 1.063554e-06

> c(AIC(pweipd), AIC(pweipq))
[1] 970.4960 971.9844

The model treating dose as a discrete factor has a slightly higher log-likelihood but according to Akaike, the improvement in fit is not worth the loss of parsimony incurred by adding two parameters.

[4] Survival Probabilities

(a) Use the piece-wise exponential model of part 3.b with effects of clinic, dose, and prison history, to predict the probability of remaining in treatment after one, two and three years for someone with no prison record receiving the average dose of 60.4 mg of methadone in clinic 1.

I will do the calculation from first principles. Because the effect of clinic 1 is not proportional we need to add it separately for each year:

. estimates restore model3b
(results model3b are active now)

. mata 
───────────────────────────────────────────────── mata (type end to exit) ──────
:     b = st_matrix("e(b)")

:     logh = b[12] :+ b[1..6]

:     c = b[(7, 7, 7, 8, 8, 9)]

:     h = exp(logh + c :+ (60.4 * b[11]))

:     H = runningsum(h :* w)

:     exp(-H)[(3, 5, 6)]
                 1             2             3
    ┌───────────────────────────────────────────┐
  1 │  .6241612559   .2497116013   .0112137274  │
    └───────────────────────────────────────────┘

: end    
────────────────────────────────────────────────────────────────────────────────
> b <- coef(pweipd)

> logh <- b[1] + c(0, b[2:6])

> xb <- b["dose"] * 60.4

> h <- exp(logh + b[c(7, 7, 7, 8, 8, 9)] + xb)

> H <- cumsum(h * w)

> exp(-H)
                  interval(3,6]  interval(6,12] interval(12,18] interval(18,24] 
     0.91498838      0.81733685      0.62416125      0.41565387      0.24971160 
interval(24,36] 
     0.01121373 

The probabilities are 62.4%, 25.0% and 1.1%.

(b) Repeat the calculations for someone with a prison record who receives 60.4 mg in clinic 1.

A prison record affects the hazard at every duration proportionally, so the calculation is simple

. mata 
───────────────────────────────────────────────── mata (type end to exit) ─────
:     hp = h :* exp(b[10])

:     Hp = runningsum(hp :* w)

:     exp(-Hp)[(3, 5, 6)]
                 1             2             3
    ┌───────────────────────────────────────────┐
  1 │    .50849745   .1365953803   .0015912996  │
    └───────────────────────────────────────────┘

: end    
────────────────────────────────────────────────────────────────────────────────
> Hp <- H * exp(b["prison"]) 

> exp(-Hp)
                  interval(3,6]  interval(6,12] interval(12,18] interval(18,24] 
      0.8803158       0.7487068       0.5084974       0.2837597       0.1365954 
interval(24,36] 
      0.0015913 

The probabilities are 50.8%, 13.7% and 0.2%.

(c) Finally, repeat the calculations for someone without a prison record who receives the average dose of 60.4 mg, but in clinic 2.

Without a prison record and clinic 2 all we need is the baseline hazard and the dose effect

. mata
───────────────────────────────────────────────── mata (type end to exit) ──────
:     h2 = exp(logh :+ (60.4 * b[11]))

:     H2 = runningsum(h2 :* w)

:     exp(-H2)[(3, 5, 6)]
                 1             2             3
    ┌───────────────────────────────────────────┐
  1 │  .7422537173   .6053897439   .5185560265  │
    └───────────────────────────────────────────┘

: end
────────────────────────────────────────────────────────────────────────────────
> h2 <- exp(logh + xb)

> H2 <- cumsum(h2 * w)

> exp(-H2)
                  interval(3,6]  interval(6,12] interval(12,18] interval(18,24] 
      0.9453671       0.8802485       0.7422537       0.6780620       0.6053897 
interval(24,36] 
      0.5185560 

The probabilities are 74.2%, 60.5% and 51.9%.

To present the results in a single table we combine all the results

. mata HA = H \ Hp \ H2

. mata exp(-HA)[ ,(3, 5, 6)]
                 1             2             3
    ┌───────────────────────────────────────────┐
  1 │  .6241612559   .2497116013   .0112137274  │
  2 │    .50849745   .1365953803   .0015912996  │
  3 │  .7422537173   .6053897439   .5185560265  │
    └───────────────────────────────────────────┘
> exp(-cbind(H, Hp, H2))[c(3, 5, 6), ]
                         H        Hp        H2
interval(6,12]  0.62416125 0.5084974 0.7422537
interval(18,24] 0.24971160 0.1365954 0.6053897
interval(24,36] 0.01121373 0.0015913 0.5185560

There are fairly substantial survival differences among these groups. With an average dose of methadone and no prison record, we estimate that more than half the addicts in clinic 2 would still be under treatment after three years, compared to just about one percent of those in clinic 1, a figure that is further reduced to less than one-fifth of one percent for addicts in clinic 1 and with a prison record.