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. Author manuscript; available in PMC: 2012 Sep 28.
Published in final edited form as: Ann Intern Med. 2007 Jan 16;146(2):133–141. doi: 10.7326/0003-4819-146-2-200701160-00008

Table 2.

Parameters in Computer Simulation*

Parameter Data Source (Reference) Strength of Evidence
Does Data Source Meet Strict Evidence Criteria?
Parameter Distribution if Data Source Used Parameter Distribution if Data Source Not Used
Study Design Internal Validity External Validity Study Design Level 1 Internal Validity Good External Validity High All 3 Criteria
Mortality rate in absence of HIV Observational; life tables (12) 1 Good Low Yes Yes No No Point estimates, variable Uniform (0.5X, 1.5X estimates)

Mortality rate attributable to HIV Observational; 1 study in similar population (13) 1 Good Low Yes Yes No No Normal (0.19, 0.06) Uniform (0, 0.38)

Impact of HIV treatment on mortality Observational; 13 studies pooled from similar populations (14) 1 Good High Yes Yes Yes Yes Normal (0.15, 0.02) NA

Probability of taking HIV medications Observational (15) 1 Fair Low Yes No No No Never used NA
Observational (16) 1 Good High Yes Yes Yes Yes Normal (0.75, 0.02) NA
Observational (17) 1 Good Low Yes Yes No No Never used NA
Observational (18) 1 Good Low Yes Yes No No Never used NA

Effectiveness of DOT Randomized, controlled trial; 1 study in dissimilar population (19) 1 Good Low Yes Yes No No Normal (0.46, 0.01) Uniform (0, 2)

Utility with HIV Observational; 1 study in similar population (20) 1 Poor Low Yes No No No Normal (0.87, 0.04) Uniform (0.5, 1.0)

Decrement in utility with HIV treatment Observational; 1 study in similar population (unpublished) 1 Poor Low Yes No No No Normal (0.05, 0.01) Uniform (0, 0.5)

Annual cost of DOT Expert opinion 3 Poor Low No No No No Never used NA
Observational; 1 study in dissimilar population (21) 2-2 Good Low No Yes No No Point estimate $4600 Uniform ($200, $36 500)
Observational; 1 study in dissimilar population (22) 2-2 Poor Low No No No No Never used NA
Observational; 1 study in dissimilar population (23) 2-2 Poor Low No No No No Never used NA

Annual cost of nondrug HIV care Observational; 1 study in similar population (24) 1 Poor High Yes No Yes No Normal ($9000, $300) Uniform ($200, $20 000)
Observational; 1 study in similar population (25) 1 Poor High Yes No Yes No Never used NA

Annual cost of HIV drugs Observational; 1 study in similar population (25) 1 Good High Yes Yes Yes Yes Normal ($10 300, $700) NA
*

Individual data sources were eligible to inform parameter estimations if their strength of evidence met or exceeded criteria in 3 separate domains (study design, internal validity, and external validity). If no data sources met strength of evidence criteria, a uniform distribution across a wide plausible range was substituted. When more than 1 study met strength of evidence criteria, the parameter’s distribution was based on the study with the most precise statistical estimate. Normal = normal distribution (mean, standard deviation); uniform = uniform distribution (lower bound, higher bound). DOT = directly observed therapy; NA = not applicable because data source either was never used or met all 3 of the strict evidence criteria.

“Never used” because results were statistically less precise than those of another study or studies with equal or superior grades of evidence in all 3 domains. It is also possible that a particular data source could be statistically more precise but have lower strength of evidence than alternative data sources, although this situation did not arise with our example. In accord with our decision rules, we would have used the statistically more precise data source when evidence criteria were more inclusive and the statistically less precise data source when evidence criteria were less inclusive.