The economic model was constructed in Microsoft Excel®(Microsoft Corporation, Redmond, WA, USA)
and uses a simple cohort model to capture changes in outcomes over time. It uses a cycle length of 1 month; no half-cycle correction was applied in the calculation of outcomes given the short cycle length relative to the time horizon. In line with the NICE methods guide,129the decision model uses a discount
rate of 3.5% per annum for both costs and benefits.
As explained in Model development, the model structure is based on cognitive ability (IQ) and adaptive behaviour (VABS) scores which are used to predict per cycle benefits (QALYs) and costs. Changes in the outcome measures (cognitive ability and adaptive behaviour) are incorporated as mean changes in each measure per month (Figure 22). Changes in outcomes are based on the IPD meta-analysis; see Treatment effect for details. As outcome data included in the IPD meta-analysis were available only at 1 and 2 years, a simple linear interpolation was used to estimate between-group differences at intervening time points.
ECONOMIC ANALYSIS
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The time horizon in the main analysis is divided into two phases, representing different stages of an individual’s childhood. The first phase ‘preschool’ (up to age 4.5 years) reflects the period when early interventions are delivered, whereas the second ‘school age’ phase covers the period in which children attend primary (age 4.5–11.5 years) and secondary education (age 11.5–18.5 years). To accommodate educational placement and the associated resource implications, the model distinguishes between three education settings: (1) mainstream, (2) mainstream with support and (3) special education provision. Estimation of the proportion of children receiving each type of education is based on cognitive ability and adaptive behaviour; see Education for details.
Scenario analysis exploring adult outcomes
Although the main (base-case) analysis limits the time horizon to childhood (15.5 years), an exploratory scenario analysis considered the potential impact of incorporating adult outcomes into the model. In this scenario, an additional phase is added to the model structure. The adult phase of the model is a distinct phase in which a different model structure is used. This was done because the costs of care for autistic adults are very different from those for autistic children.
In several of the economic evaluations identified in the cost-effectiveness review (see Appendix 13), adult outcomes were defined in terms of levels of independence, often with three levels defined (e.g. independent, semi-independent and dependent). The definitions attached to each of these levels varied across the
different models, but generally adopted one of two approaches. In the first most commonly used approach, levels of independence were defined with reference to special education and in some cases special education needs were also used to predict adult outcomes. The second approach used evidence from the epidemiological and observational literature, which has categorised adult outcomes based on normative evaluations of independence and social outcomes (see Appendix 13 for an overview of these studies). This approach has important strengths, as it focuses on the support an individual needs and therefore the categorisation systems are useful in terms of reflecting costs. For example, Penner et al.54use a
classification system reported in Howlin et al.,15which is widely adopted within the epidemiological and
observational literature. This classification system is useful because it describes a number of important elements of care, with the definitions for each category, including references to residential status and daily needs, both of which are significant drivers of cost in adulthood. In the context of the Penner et al.54model, this also allowed them to link the short-term outcomes reported in the effectiveness
studies to long-term adult outcomes.
This approach, however, has important limitations, as several studies have noted poor correlations between HRQoL and indicators of independence in autistic populations.130Furthermore, a number of
studies have been critical of this type of categorisation of adult outcomes, noting that a more nuanced approach is needed to best define ‘standard concepts of what constitutes a “good” social outcome, [as these] may not always be relevant for people with ASD’.131
Adaptive behaviour Cognitive ability Adaptive behaviour Initiation of therapy Death Adulthood Secondary school Primary school Preschool Cognitive ability Adaptive behaviour Cognitive ability Adaptive behaviour Adult outcomes Educational placement Educational placement Cognitive ability
The structure adopted in the adult scenario was therefore based on the classification system developed in Howlin et al.,15similar to the approach used in Penner et al.54However, noting the cited limitations,
and in an attempt to mitigate these, independence levels are only used to indicate care needs and costs in adulthood. HRQoL, instead, continues to be based on the Payakachat et al.127algorithm used to
estimate quality of life in children.
The structure adopted in the adult phase to estimate costs is based on five levels of independence: (1) ‘completely independent’, (2) ‘mostly independent’, (3) ‘some independence’, (4) ‘mostly dependent’ and (5) ‘completely dependent’. The definitions used for each category are based on those used in Howlin et al.15and are described in Table 9, and the costs attached to each independence level are
described in Resource use.
Independence levels are determined on entering the adult phase of the model based on adaptive behaviour scores at 18.5 years of age. Owing to limitations in the epidemiological and observational evidence base, the adult phase of the model is static (i.e. once individuals enter the adult part of the model, the same level of independence is assumed throughout adulthood). This simplifying assumption is made because of lack of data regarding changes in independence over time. This may or may not reflect real life, and does not include any consideration of an individual’s potential for change or the impact of any additional health or mental health needs, life events or changes in the wider family or social community. This assumption therefore necessarily represents an important limitation of this scenario.