Averett 2012.

Methods	Controlled before‐and‐after study; triple differences with state and individual fixed‐effect method; 1992‐1998
Participants	3365 participants of 2 waves of the National Longitudinal Survey of Youth 1979 cohort (1992, 1998); working‐age adults (27 to 41 years in 1992); women; African‐American (1404 participants) and European‐American (1961 participants); US
Interventions	Increase in value of the Earned Income Tax Credit in 1996
Outcomes	Primary outcomes: tobacco use Secondary outcomes: change in employment
Notes	Intervention context: permanent in‐work tax credit intervention for families introduced in 1975; means‐tested to low‐income groups (phase out starts at 17% to 42% of average income from wages (depending on family type); Immervoll 2009); amongst the most generous in‐work tax credit interventions internationally (up to 7% of an average income from wages for families with 1 dependent child; up to 11% of an average income from wages for families with 2 dependent children; Immervoll 2009); distributed nearly USD 62 billion to over 27 million individuals in 2011; had a high (approximately 80%) up‐take and low (less than 1%) administration costs; follow‐up for intervention to outcome measure was 2 years (1996‐1998). Equity impact estimated for the following PROGRESS variables: level of education
Risk of bias
Bias	Authors' judgement	Support for judgement
Allocation concealment (selection bias)	Unclear risk	Unclear whether random sampling strategy employed; nationally representative sample achieved
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Survey participants not allocated to the intervention by the researchers; secondary analysis of survey data collected for a different purpose than estimating the impact of in‐work tax credit for families on health in adults (that is, blinding of participants and personnel neither feasible nor necessary)
Blinding of outcome assessment (detection bias) All outcomes	High risk	Survey participants not allocated to the intervention by the researchers; secondary analysis of survey data collected for a different purpose than estimating the impact of in‐work tax credit for families on health in adults (that is, blinding of outcome assessors neither feasible nor necessary); high risk from misclassification bias of the exposure due to crude exposure assessment
Incomplete outcome data (attrition bias) All outcomes	High risk	Survey response rate at wave one not reported; observations with missing information on key variables excluded from the analysis (complete case analysis); high risk of bias from attrition (59.9%)
Selective reporting (reporting bias)	Low risk	Significant and non‐significant effects reported
Other bias	High risk	The treatment and control groups differed in health measures and characteristics at baseline; key confounders reported and controlled for robustly (except employment status); adjustment for income (a potential mediator of the in‐work tax credit‐health relationship) could have biased the results towards the null; reverse causation not controlled for