Fiona Imlach Gunasekara

Change in income and change in self-rated health: Systematic review of studies using repeated measures to control for confounding bias

It is generally assumed that income is strongly and positively associated with health. However, much of the evidence supporting this assumption comes from cross-sectional data or analyses that have not fully accounted for biases from confounding and health selection (the reverse pathway from health to income). This paper reports results of a systematic review of panel and longitudinal studies investigating whether changes in income led to changes in self-rated health (SRH) in adults. A variety of electronic databases were searched, up until January 2010, and thirteen studies were included, using data from five different panel or longitudinal studies. The majority of studies found a small, positive and statistically significant association of income with SRH, which was much reduced after controlling for unmeasured confounders and/or health selection. Residual bias, particularly from measurement error, probably reduced this association to the null. Most studies investigated short-term associations between income and SRH or the effect of temporary (usually one year) income changes or shocks, so did not rule out possibly stronger associations between health and longer-term average income or income lagged over longer time periods. Nevertheless, the true causal short-term relationship between income and health, estimated by longitudinal studies of income change and SRH that control for confounding, may be much smaller than that suggested by previous, mostly cross-sectional, research.

What is the association between wealth and mental health

Background: Socioeconomic inequalities in mental health have been shown in a number of populations. This study aims to investigate the association between asset wealth and psychological distress in New Zealand and whether it is independent of other socioeconomic measures and baseline health status. Methods: Data for this study were from the first three waves of the Survey of Families, Income and Employment (SoFIE) conducted in New Zealand (2002–2004/05) (n = 15 340). The Kessler-10 was used as a measure of psychological distress. The association of quintiles of wealth with psychological distress was investigated using logistic regression, controlling for confounders, socioeconomic variables and prior health status. Results: The odds ratio (OR) of reporting high psychological distress were greater in the lowest wealth quintile compared with the highest (OR 3.06, 95% CI 2.68 to 3.50). Adjusting for age and sex did not alter the relationship; however, adjusting for income and area deprivation attenuated the OR to 1.73 (95% CI 1.48 to 2.04). Further controlling for baseline health status reduced the OR to 1.45 (95% CI 1.23 to 1.71), although the confidence interval still excluded the null. Conclusions: Inequalities in wealth are strongly associated with psychological distress, over and above other confounding demographic variables and baseline health status. Much, but not all, of that association is confounded by adult socioeconomic position. This suggests that policy measures to improve asset wealth, through savings and home ownership, may have positive health implications and help to reduce health inequalities.

Comparing self-rated health and self-assessed change in health in a longitudinal survey: Which is more valid?

Self-rated health (SRH) is commonly used in longitudinal analyses as a repeated outcome measure. This assumes that computed changes in SRH over time truly represent within-individual changes in underlying health. The longitudinal validity of SRH, however, is threatened by ceiling effects (where people reporting the highest level of SRH cannot report subsequent improved health), insensitivity to small changes within SRH categories, reference group effects (where individuals assess their health changes relative to their peers) and stability in SRH even when change in underlying health is occurring. We assessed the longitudinal validity of SRH by comparing computed changes in SRH with a measure of self-assessed change in health (SACH). We used two waves of data (2003-2005) from the New Zealand longitudinal Survey of Family, Income and Employment (SoFIE). Computed change in SRH and SACH were compared directly and also in regression models using an objective measure of health outcome change (hospitalisations within the past year). Computed change in SRH and SACH were not well correlated, consistent with ceiling and/or categorisation effects in SRH. In regression models, SACH was more strongly predictive of hospitalisation than computed change in SRH (worse SACH was associated with an increased odds of hospitalisation of 3.7 compared to 1.8 for decreased computed change in SRH). SACH may be affected by recall bias, but if SRH is used as a repeated outcome measure in longitudinal analyses, results may also be biased, if change in SRH does not occur in response to significant health events.

The relationship between income and health using longitudinal data from New Zealand

Background Evidence for a cross-sectional relationship between income and health is strong but is probably biased by substantial confounding. Longitudinal data with repeated income and health measures on the same individuals can be analysed to control completely for time-invariant confounding, giving a more accurate estimate of the impact of short-term changes in income on health. Methods 4 years of annual data (2002–2005) from the New Zealand longitudinal Survey of Family, Income and Employment were used to investigate the relationship between annual household income and self-rated health (SRH) using a fixed-effects ordinal logistic regression model. Possible effect modification of the income–SRH relationship by poverty and baseline health was tested with interactions. Results An increase in income of

Do changes in socioeconomic factors lead to changes in mental health? Findings from three waves of a population based panel study

10 000 over the past year increased the odds of reporting better SRH by 1% (OR 1.01, 95% CI 1.00 to 1.02). Poor baseline health significantly modified the association between income and SRH. A

Income and individual deprivation as predictors of health over time

10 000 increase in income increased the odds of better SRH by 10% for those with two or more chronic conditions. Poverty or deprivation did not modify the income–health association. Conclusions The overall small, positive, but statistically non-significant, income–health effect size is consistent with similar analyses from other longitudinal studies. Despite the overwhelming consensus that income matters for health over the medium and long-term, evidence free of time-invariant confounding for the short-run association remains elusive. However, measurement error in income and health has probably biased estimates towards the null.

Income-related health inequalities in working age men and women in Australia and New Zealand

Background There has been little investigation of changes in socioeconomic measures and mental health (MH)/illness over time within individuals using methods that control for time-invariant unobserved confounders. We investigate whether changes in multiple socioeconomic measures are associated with self-reported MH using fixed effects methods to control for unobserved time-invariant confounding. Methods Data from three waves of a panel study with information on MH, psychological distress, labour force status, household income, area and individual deprivation. Fixed effects regression modelling was used to explore whether changes in socioeconomic exposures were associated with changes in MH. We also compared increases and decreases in exposure with changes in MH using first difference models. Results Respondents who moved into inactive labour force status experienced a 1.34 unit (95% CI −1.85 to −0.82) decline in SF-36 MH score and a 0.50 unit (95% CI 0.34 to 0.67) increase in psychological distress score. An increase in individual deprivation was associated with a 1.47 unit (95% CI −1.67 to −1.28) decline in MH score and a 0.57 unit (95% CI 0.51 to 0.63) increase in psychological distress. Increasing and decreasing levels of individual deprivation were associated with significant changes in both outcomes. Conclusions This paper suggests that moving from employment to inactivity and changes in levels of individual deprivation may be more important for short-term MH outcomes than changes in household income or area deprivation. Providing short-term social and economic support for those experiencing financial/material hardship should be considered in interventions to reduce inequalities in MH.

The impact of in-work tax credit for families on self-rated health in adults: a cohort study of 6900 New Zealanders

ObjectivesPoverty, often defined as a lack of resources to achieve a living standard that is deemed acceptable by society, may be assessed using level of income or a measure of individual deprivation. However, the relationship between low income and deprivation is complex—for example, not everyone who has low income is deprived (and vice versa). In addition, longitudinal studies show only a small relationship between short-term changes in income and health but an alternative measure of poverty, such as deprivation, may have a stronger association with health over time. We aim to compare low income and individual deprivation as predictors of self-rated health (SRH), using longitudinal survey data, to test the hypothesis that different measures of poverty may have different associations with health.MethodsWe used three waves from the longitudinal Survey of Family, Income and Employment and fixed-effect linear regression models to compare low income (<50% median income at each wave) and deprivation (reporting three or more items from the New Zealand individual deprivation index) as predictors of SRH (coded 1–5; SD 1.1–1.2). We also compared the impact of duration of low income and deprivation on SRH using mixed linear models.ResultsIn the fixed-effect models, moving into deprivation between waves was associated with a larger decline in SRH compared to moving into low income, which persisted in models including both low income and deprivation. Similar findings were observed for duration of low income and deprivation in mixed models.ConclusionsMoving into high levels of individual deprivation is a stronger predictor of changes in SRH than moving into low income. When investigating the association of hardship poverty with health, using alternative measures, in addition to income, is advisable.

Health shocks adversely impact participation in the labour force in a working age population: a longitudinal analysis.

Objective : To examine income‐related inequalities in health in working age men and women in Australia and New Zealand.

P1-409 Differential loss of participants does not necessarily cause selection bias

Background In-work tax credit (IWTC) for families, a welfare-to-work policy intervention, may impact health status by improving income and employment. Most studies estimate that IWTCs in the USA and the UK have no effect on self-rated health (SRH) and several other health outcomes, but these estimates may be biased by confounding. The current study estimates the impact of one such IWTC intervention (called In-Work Tax Credit) on SRH in adults in New Zealand, controlling more fully for confounding. Methods We used data from seven waves (2002–2009) of the Survey of Family, Income and Employment, restricted to a balanced panel of adults in families. The exposures, eligibility for IWTC and the amount of IWTC a family was eligible for, were derived for each wave by applying government eligibility and entitlement criteria. The outcome, SRH, was collected annually. We used fixed effects regression analyses to eliminate time-invariant confounding and adjusted for measured time-varying confounders. Results Becoming eligible for IWTC was associated with no detectable change in SRH over the past year (β=0.001, 95% CI −0.022 to 0.023). A