having a resilient health system, as well informing policy about the required investment in health care workers.

# **The Relationship between Health Care Workers and Health Outcomes**

A number of studies have attempted to assess the relationship between health care workers and health outcomes. Almost all of these rely on regressions of cross-sectional data, but they vary greatly in which outcome variables (different mortality rates, vaccine coverage, or coverage of births by skilled attendants), explanatory variables (density of health care workers, doctors, or nurses and midwives), and controls they include (poverty, GDP, education), as well as in the functional forms used in their econometric analysis (logit-log, log-linear, linear regressions with arcsin and log transformation of the dependent and independent variables, logit-log and arcsinelog model), not to mention in their results.<sup><u>11</u></sup> We focus our attention on those studies investigating the effect of health care worker density (i.e., the number of health care workers per 1,000 of the population) on mortality, as opposed to alternative health outcomes. The relationship between the health care workforce and mortality is more studied than other outcomes and serves as a proxy for the overall quality of the health sector.

Early studies found no significant association between doctor density and infant mortality,<sup><u>12, 13</u></sup> or even an adverse association (i.e., positive) between the doctor density, and infant and perinatal mortality.<sup><u>14</u></sup> However, more recent studies, with access to more extensive and better quality data, have consistently found a negative and a significant association between the density of health care workers and mortality.

In this paper we focus on five of these recent studies. These studies take a range of approaches, which are summarized in Table A1. One key area of difference in the approaches is how they treat health workers: either as an aggregate, or looking at the effects of doctors, nurses, and midwives separately (possibly allowing for an interaction between doctors and nurses). Robinson and Wharrad use data from 155 countries and find a negative relationship between doctor density and maternal, infant, and under-five mortality. However, they find no statistically significant relationship for nurses once they are included with doctors.<sup><u>15, 16</u></sup> Anand and Bärninghausen use data from 117 countries and find a significant negative association between the density of aggregate health care workers (doctors, nurses, and midwives combined) and maternal, infant, and under-five mortality. They also find a significant negative relationship between doctor density and all mortality rates, while the coefficient for nurses is insignificant once the controls are included. <u>17</u> Speybroeck et al. use data for 192 countries. They find a significant negative relationship between aggregate health care worker density and maternal mortality, with a similar elasticity to that of Anand and Bärninghausen, but unlike the latter they find the coefficients for infant and under-five mortality to be insignificant. In the case of disaggregate densities – where they are unique in their inclusion of an interaction term between doctor density and the density of nurses 4