Table 6.

The association in the population of selected individuals differs from the causal association in the target population. Hernán (2017) calls this scenario ‘selection bias off the null’. Lu et al. (2022) call this scenario ‘Type 2 selection bias’. We call this bias ‘target population restriction bias at baseline’

Target Population Restriction Before Start of Study
Bias		Causal Graph
1	Problem: Target population is not WEIRD; sample population is WEIRD
	Response: Sample from the target population (with assumptions)
2	Problem: Target population is WEIRD; sample population is not WEIRD
	Response: Eligibility restrictions
3	Problem: Correlated measurement error of treatment and outcome in an overly ambitious target population
	Response: Eligibility restrictions for a less ambitious target
4	Problem: Correlated measurement error of measured effect-modifiers for an overly ambitious target population
	Response: Eligibility restrictions for a less ambitious target population

Key: A denotes the treatment; Y denotes the outcome; asserts causality; indicates conditioning on variable X; indicates fixing co-variate to level X = 1; indicates effect modification of by F; biased path for treatment effect arising from confounding by a common cause; biased path for treatment effect in target population; indicates a latent variable X measured by proxy X′; indicates that conditioning on X introduces bias.