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Table 2 Types of clinical covariates suggested across all resources

From: Investigating clinical heterogeneity in systematic reviews: a methodologic review of guidance in the literature

General Category Specific Covariate Number of Resources Recommending1
Patient level General2 15
  Age 7
  Baseline severity 3
  Sex/gender 4
  Ethnicity 2
  Comorbidities 2
  Other disease features 2
Intervention level General 13
  Dose 8
  Duration 5
  Brand 3
  Co-interventions 3
  Intensity 3
  Timing 3
  Route 2
  Compliance 2
  Others unique to the intervention 2
  Frequency 1
  Comparator/control 1
Outcome level General 6
  Event type 5
  Length of follow-up 4
  Outcome measure type 3
  Outcome definition 3
  Timing 2
  Repeated outcome measurements 1
Control event rate / baseline risk 14
Research setting 4
Comparison conditions 3
Early stopping rules 1
Population risk 1
  1. 1. The number (N) of resources equals the percentage of resources since we include 101 total resources.
  2. 2. The term “general” means that the resource listed the term “patient”, “intervention”, or “outcome” as a category from which to consider covariates without suggesting specific variables.