Epidemiology is about measuring disease or other aspects of health in populations, identifying the causes of ill-health and intervening to improve health, and we come back to these three fundamental components later in the chapter. But what do we mean by ‘health’? Back in 1948, the World Health Organization defined it as ‘… a state of complete physical, mental and social well-being’ (WHO, 1948). In practice, what we usually measure is physical health, and this focus is reflected in the content of most routine reports of health data and in many of the health measures that we will consider here; however, there are now methods to capture the more elusive components of mental and social well-being as well. Importantly, the WHO recognised that it is not longevity per se that we seek, but a long and healthy life. So, instead of simply measuring ‘life expectancy’, WHO introduced the concepts of ‘health-adjusted life expectancy’ (HALE) and subsequently ‘disability-adjusted life years’ (DALYs) to enable better international comparisons of the effectiveness of health systems.

The overarching goal of public health is to maximise the health of the population, and to achieve this we need evidence about what works and what does not work. Good studies are difficult to design and implement, and interpretation of their results and conclusions is not always as straightforward as we might hope. How, then, can we make the best use of this information? In the next three chapters we look at ways to identify, appraise, integrate and interpret the literature to generate the evidence we need to inform policy and practice. In this chapter we focus on interpreting the results from a single study, because if they are not valid they will be of limited value. The central question we have to answer when we read a study report is, ‘Are the results of the study valid?’

The author’s Complementarity Principle states that bilinguals usually acquire and use their languages for different purposes, in different domains of life, with different people. Different aspects of life require different languages. The author first evokes the contribution of past research on his thinking about the topic. For instance, early sociolinguistic studies of bilingual communities put heavy emphasis on the functions of languages. He then concentrates on the principle itself as it slowly came into focus in his work. He defines it, presents it in a diagram, and discusses its impact on language proficiency, dominance, and translation abilities in bilinguals. This is followed by a description of studies undertaken in his laboratory that found evidence for it, as well as his search for studies in production, perception, and language acquisition that confirmed its importance. The author then discusses the coming of age of the principle reflected in the reactions of researchers in the field, and the various studies, mostly experimental and descriptive, they have conducted to examine it. The principle is one of the most pervasive aspects of individual bilingualism, and it is invariably present in bilingual psycholinguistic research as an independent, control, or confounding variable.