Expresión oral

Around 4% of the final sample did not have enough information to construct an instrumental variable and therefore was excluded in the IV analyses (psychotherapy use for

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MDD patients: 137 missing; psychotherapy use for the previous patient: 133 missing). When test the two IVs separately in the first-stage logistic regression, the results indicated that both of our IVs were strongly associated with psychotherapy use (psychotherapy use for MDD patients, z= 7.69, p< 0.001; chi-square = 59.15, p< 0.001; psychotherapy use for the previous patient z= 14.84, p< 0.001; chi- square= 220.24, p< 0.001). When both IVs were included in the first-stage regression model, both of our IVs were still significantly associated with psychotherapy use although the strength of psychotherapy use for MDD patients became weaker (z= 2.44, p< 0.015 for psychotherapy use for MDD patients; z= 12.29, p< 0.001 for psychotherapy use for the previous patient). The Wald test result showed that the two IVs were jointly significant in the first-stage model (chi-square= 218.96, p<0.001). Since both IVs seemed to be strongly associated with the use of psychotherapy, the IV selection was then mainly based on the exclusion restriction criterion. Table 4.20 summarizes the IV selection for each dependent variable based on the results of Hausman tests, and the specification tests results are shown in Appendix 5.

Table 4.20. IV Selection for Each Dependent Variable Psychotherapy use

for MDD patients

Psychotherapy use for the previous patient

Medication discontinuation X Medication Switching X X Hospitalizations part 1 model X part 2 model X Inpatient costs part 1 model X part 2 model X X Outpatient costs X

Prescription drug costs X X

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Notice that except for four dependent variables: medication switching, the second part of the hospitalizations, the second part of the inpatient costs, and total treatment costs, at least one IV can be considered as valid instrument for all other dependent variables using

Hausman tests. Because of negative chi-squared statistics, whether the two IVs meet the exclusion restriction from Hausman tests was unable to be determined for these four outcome models. The decision of including both IVs in these two models was instead based on the results from Chow tests as well as the LR test which indicated that both IVs should be validly excluded from the second-stage model. While for the second-part hospitalization model and total cost model, we only included the use of psychotherapy for the previous patient in the model because the LR tests reject the null that both IVs were validly excluded from the second-stage of the IV model. Since the first-stage test showed that the use of psychotherapy for the previous patient was a stronger IV than psychotherapy use for MDD patients, only the use of psychotherapy for the previous patient was used as an IV in the second-stage model.

Finally, because the Hausman test for the endogeneity of psychotherapy did not provide valid test results for all of the outcome models, using a formal test to check whether psychotherapy was endogeneous was not feasible. Therefore, the use of adjunctive

psychotherapy was assumed to be endogeneous with respect to all of the outcomes, and IV estimates were constructed for all the outcome models. The results from the naïve regression models and IV models are summarized in Table 4.21.

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Table 4.21. Point Estimates and Confidence Intervals of Adjunctive Psychotherapy Use

Original Model IV model

β 95% CI β Bootstrap 95% CI Medication discontinuation† -0.59* (-1.02, -0.17) -0.53* (-0.97, -0.07) Medication switching† 0.45* (0.25, 0.66) 0.48* (0.20, 0.69) Hospitalizations part 1 model 0.26* (0.03, 0.48) 0.23 (-0.07, 0.49) part 2 model 0.30 (-0.56, 0.66) 0.13 (-0.55, 0.44) Inpatient costs part 1 model 0.24* (0.01, 0.47) 0.23 (-0.08, 0.49) part 2 model 0.86* (0.62, 1.09) 0.72* (0.28, 0.96) Outpatient costs 0.36* (0.10, 0.62) 0.42* (0.14, 0.73) Antipsychotic costs 0.13* (0.04, 0.23) 0.37* (0.06, 0.68) Total costs 0.39* (0.19, 0.59) 0.42* (0.14, 0.73) CI: Confidence Interval

† Both medication persistency and medication switching were measured using a 30-day gap/window definition * Significant at α= 0.05 level

In Table 4.21, we can see that the point estimates of psychotherapy use do not change much after applying the IV technique. The effect of psychotherapy on most outcome

variables increased in the IV models compared to the estimates in the naïve models with the exception of coefficients in the discontinuation, hospitalization, and inpatient cost models. Use of adjunctive psychotherapy significantly decreased the likelihood of discontinuation at the beginning of the follow-up. Unlike the results from the naïve regressions, psychotherapy use did not significantly increase the likelihood of hospitalization. However, according to the second-stage IV inpatient cost model, among patients who had been admitted, the use of psychotherapy led to an 2.05 [exp(0.72)= 2.05] times increase in inpatient costs. After combining both parts of the inpatient cost models and calculating the unconditional incremental costs, we found patients with psychotherapy still had significantly higher inpatient costs (mean incremental cost= $1,045.43, 95% CI= $513.63-$1,652.30) although

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this result was slightly lower than the result from the original model without IVs (mean = $1,390.13, 95% CI= $803.35-$1,927.39).

Similar to the naïve estimates, results from the IV model indicated that the use of adjunctive psychotherapy increased outpatient costs by 52% [exp(0.42)= 1.52]. After accounting for the potential endogeneity, the IV estimates showed that psychotherapy use caused a 45% [exp(0.37)= 1.45] increase in antipsychotic costs compared to a 14%

[exp(0.13)= 1.14] increase from the naïve estimate. Finally, the IV estimate suggested that psychotherapy increased the total treatment costs by a factor of 1.52 [exp(0.42)= 1.52] which was slightly higher than the naïve estimate [exp(0.39)= 1.48].

When comparing the IV estimates to the naïve estimates, it is important to know that IV provides the local average treatment effect (LATE), which estimates the effect of

psychotherapy on marginal patients (i.e. patients can be treated either with or without psychotherapy), while the naïve model estimates average treatment effect as the difference between the average treatment effect as treated and the average treatment effect as non- treated, which compares the effect of psychotherapy between those who received it versus who did not.145-147 Therefore, the differences between IV estimates and naïve estimates may be caused by different populations being used for the estimations, and both estimates could be accurate. On the other side, since 1) the naïve models could suffer from un-measurable confounding (as discussed in section 3.5), and 2) our IVs could still be correlated with the outcomes through unobservable routes (as discussed in section 3.6.3) and thus bias the IV results, it is possible that both our naïve estimates and IV estimates are biased.

Finally, except for the second-part of the hospitalization model, outpatient costs, and total treatment costs, most of our results failed to reject the null hypothesis that the use of

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psychotherapy was exogeneous. This indicates that the use of psychotherapy may not be endogeneous with respect to most of our outcomes. However, as discussed before, the exogeneity test is inconclusive. We may fail to reject the null because of weak or invalid instruments. Given the high variance in the second-stage model, we often fail to reject the null because of low power. Even if the null hypothesis is rejected, it can be caused by measurement errors or incorrect functional form. As a result, failing to reject the null does not conclude the exogeneity of psychotherapy use. We should treat these test results as references, and keep in mind that the use of psychotherapy can still be endogeneous.

4.5 Summary

In summary, only a few patients (17%) received psychotherapy during the 18-month follow-up period, and most patients had a relatively short treatment duration for

psychotherapy (mean psychotherapy visits= 5.4 and mean treatment duration= 160 days). Our naïve estimates showed that the use of adjunctive psychotherapy was associated with better antipsychotic persistency at the beginning of the follow-up. However, this protective effect diminished within the first two months of follow-up, and patients with psychotherapy became less persistent than patients without psychotherapy after the first two months. Patients with adjunctive psychotherapy were also found to be more likely to switch their antipsychotics and had higher treatment costs. However, we found the use of adjunctive psychotherapy was associated with a small but significant increase in hospitalizations.

Consistent with our naïve estimates, the IV estimates showed that patients receiving psychotherapy had significantly better persistency at the beginning of the follow-up. Patients with psychotherapy were still found to be more likely to switch medications, and they had

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higher outpatient, pharmacy, and total treatment costs compared to patients without

psychotherapy. In contrast, results from IV models did not show that the use of adjunctive psychotherapy leads to a higher risk of hospitalization. If our IVs are strong and truly valid, we can conclude that the use of adjunctive psychotherapy increase patients’ persistency and treatment costs, but it does not significantly increase the risk of hospitalization.