Assumption	Justification	Implication
The 1-year incidence is constant at 238,350 cases per year. Prevalence is 588,600 annually in the adult Dutch population of 10 million people.	Data obtained from the Netherlands Mental Health Survey and Incidence Study (NEMESIS), a population-based psychiatric epidemiological cohort study [15].	Prevalence determines the cost and effects. The ratio incidence/prevalence determines the relative importance of prevention.
Episode duration is 6 months on average.	After Spijker et al. [40]	Taking episode duration into account affects health benefits. A shorter duration means less potential to generate health benefits.
It is possible to have up to 5 recurring episodes of depression. After the fifth recurrence, a patient is assumed to be chronically depressed. Recurrence rates of depressive disorder are 50%, 70%, 80%, 85% and 90% for the first to the fifth episode.	Relapse rates are higher after a previous depressive episode	Increasing risk of recurrence results in patients making heavier demands on the health care system, which emphasizes the importance of preventing recurrence from a cost-effectiveness point of view.
Effects are normally distributed.	After Briggs et al. (2006) [35].	Uncertainty around the effect parameters is symmetrical
Costs are gamma distributed.	After Briggs et al. (2006) [35]	Uncertainty around the cost parameters is skewed to the right.
Costs include only direct medical costs (in this paper).	Production losses are not relevant for retired people. Direct non-medical costs are only a fraction from direct medical costs.	The model’s output is from the perspective of healthcare providers, not the patient, and not from parties such as employers.
The willingness to pay (WTP) for averting 1 DALY is €20,000 ($23,755).	WTP for averting 1 DALY can be as much as €80,000 ($95,020). A relative low number of €20,000 ($23,755) was chosen to be more conservative.	A healthcare system is deemed cost-effective when the price per DALY averted is less than the WTP ceiling of €20,000 ($23,755).
Effects of CBT are maintained over at least one year after treatment, but effects of pharmaceutical interventions decline almost instantly after discontinuation.	Based on analysis after [22]	Longer lasting prophylactic effects for CBT than for pharmaceutical interventions, amounts to in increased cost-effectiveness of CBT relative to anti-depressant medication.
CBT offered during the acute stage of depression, introduces a prophylactic effect.	Based on analysis after [22]	The presence of a prophylactic effect makes it more desirable to treat acute cases of depression with CBT because it may help to avoid new onsets of the disorder in the future.