Strully 2010.

Methods	Interrupted time series study; difference‐in‐differences with state fixed‐effect method; 1988‐2002, 1980‐2002
Participants	5,260,202 participants from 15 years of birth records (1988‐2002); working‐age adults; women; US (selected states (California, Indiana, Louisiana, Nebraska, New York, Oklahoma, South Dakota, Washington) contributed data for selected years only) 66,542 participants from 23 years of data from the March Current Population Survey (annual waves 1980‐2002); working‐age adults; women; US
Interventions	Any changes in Earned Income Tax Credit between 1988 (and 1980, respectively) and 2002
Outcomes	Primary outcomes: tobacco use Secondary outcomes: change in income; change in employment
Notes	The sample may have included a small number of non‐working age participants 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: none
Risk of bias
Bias	Authors' judgement	Support for judgement
Allocation concealment (selection bias)	High risk	Vital statistics of birth records covering close to all births in the population used; sample not nationally representative due to data from some states omitted for some years; high risk of bias from selection
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	Unclear 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	Birth registry covers close to all births in the population; high percentage of 23% of birth records missed data on the outcome; moderate percentage of participants of the March Current Population Survey missed values on one or more variables used in the analysis; observations with missing information on key variables excluded from the analysis (complete case analysis)
Selective reporting (reporting bias)	Unclear risk	Significant and non‐significant effects reported
Other bias	Unclear risk	The intervention was unlikely to alter data collection. The point of analysis was the point of intervention. The study adjusted for trends over time in the health outcome between women living in a state with an EITC (treatment group) and women residing in a state without an EITC (control group); however, it did not adjust for underlying, differential trends between the treatment and control groups, meaning that the study had a high risk of bias from unmeasured or unadjusted confounding. Reverse causation not controlled for

EITC: Earned Income Tax Credit