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Up to this point, the scientific basis for caries control and practical details for delivery of caries control to the individual have been given. We now change tack and consider caries control in populations. In order to follow the health profiles in populations there is an important tool called epidemiology. This literally means ‘the study of what is upon people’. It is derived from Greek where ‘epi’ means upon or among and ‘demos’ is people (population). In other words, epidemiology is the study of the distribution (how often) various diseases occur and why they appear in well-defined populations. It deals with groups of people, not individuals. Data thus obtained are used in public health for developing and monitoring strategies for health care in populations. Moreover, it can tell how diseases are influenced by hereditary factors, by physical and social environments, and human behaviour. All this helps health authorities to develop appropriate preventive interventions and make these as cost-effective as possible. In this chapter, having introduced the concepts of epidemiology, examples of caries control in two populations and its assessment using epidemiological measurements is given. However, the use of epidemiology has already been described in Chapter 4, where Dean’s observations on the relationship between fluoride in water supplies, the resulting dental fluorosis, and the concomitant caries reduction are described (see Chapter 4). In a recording system of any disease it is important to have clear criteria for diagnosis. The following are important: ◆ How valid are the criteria of measurement? Do they record what they are intended to measure? ◆ How reliable are the criteria? Reliability is also covered by the terms reproducibility, and consistency. These terms imply that the same or different examiners can use the criteria in the same way on different occasions and obtain the same result. ◆ The criteria should be clear, simple, and objective. In other words robust. This is particularly important if manifestations of a disease are to be grouped in different categories of severity, as with dental caries.

How tall is the human race? What is meant by being short? Walking down the street, one will see people of various heights and a degree of variation exists. Some people are shorter than others, but when is someone abnormally so? How is it possible to make this judgement? By recording the height of everyone it is possible to start to produce a picture of people as a whole. Such terms as minimum, maximum, and mean give an indication of the distribution of heights. The science used to collect and examine data in this way is known as epidemiology. Epidemiology is defined as: . . . The orderly study of diseases and conditions where the group and not the individual is the unit of interest. . . . Mausner and Kramer ( 1985 ) state that epidemiology is concerned with the frequencies of illnesses and injuries in groups of people as well as the factors that influence their distribution. By investigating differences between subgroups of the population and their exposure to certain factors it is possible to identify causal factors and consequently to develop programmes to alleviate the problems. The critical issue is that knowledge is gained by studying patterns in groups as opposed to concentrating solely on the individual. This chapter gives an overview of the uses of epidemiology in dentistry and describes the main principles of this subject. Epidemiology in dentistry operates in three broad fields. These are: . . . 1 the measurement of dental disease among groups within the population in order to understand factors that influence the distribution; . . . . . . 2 identification of factors that cause conditions; . . . . . . 3 evaluation of effectiveness of new materials and treatment in clinical trials and assessment of needs and requirements for dental services within the community. . . . Undertaking epidemiological investigations requires a series of standards and procedures; measures must be made to an agreed common standard, in a methodological manner, and, when necessary, using an appropriate random sample.

Geographic issues loom large as the American population begins the new millennium. Regional fertility differentials are growing, social networks focus new immigrants on a small number of port-of-entry metropolitan areas and states, highly channelized migration streams redistribute population in response to economic and social restructuring, and a highly variegated landscape of aging has emerged. Perhaps at no other time in its history has the field of population geography been confronted with a more intellectually important and socially relevant research agenda. Building upon its strong tradition in spatial demography and incorporating an increasingly diverse set of quantitative and qualitative methodologies, population geography today seeks a more complete understanding of human movement, regional demographic variability, and the social context within which these population processes occur. In addition, population geographers increasingly tackle issues of policy significance. After a brief review of the history of population geography and an empirical analysis of its presence in geography’s major journals, we summarize six lines of contemporary research including studies of: (1) internal migration and residential mobility; (2) international migration, transnationalism, and the nexus of internal and international migration systems; (3) immigrant assimilation, acculturation, and the emergence of ethnic enclaves; (4) regional demographic variability; (5) the social context for population processes; and (6) public policy research. We conclude by identifying major challenges facing the field today and fruitful new directions for research including the need for greater emphasis on environmental issues, integration with geography’s new technologies, and more social relevance. Although geographers long had integrated population characteristics into their broader regional studies, population geography emerged as a distinct field of study only in the early 1950s. It, like urban geography, surfaced from a discipline that was strongly rooted in the study of rural cultural landscapes and regional inventories. Its birth was marked by the 1953 AAG presidential address of Glenn Trewartha, a noted climatologist and population geographer. Trewartha lamented the neglect of population in the discipline of geography, which was at that time organized into the subdivisions of physical and cultural geography. He argued for a new threefold structure organized around population, the physical earth, and the cultural landscape.

While the physical health of women and children is emphasized in international policy guidelines, the mental dimensions of their health are often ignored, especially in developing countries. However, recent and strong evidence suggests that the mental and physical health of mothers and children is inextricably linked, and the one cannot be possible without the other (Prince et al. 2007). This chapter reviews the evidence and suggests directions for policy and research in this area. Depression is the fourth leading cause of disease burden and the largest cause of nonfatal burden, accounting for almost 12% of all total years lived with disability worldwide. Depression around childbirth is common, affecting approximately 10–15% of all mothers in Western societies (O’Hara and Swain 1996). Epidemiological studies from the developing world have reported increasingly high rates of postnatal depression in diverse cultures across the developing world. An early pioneering study by Cox (1979) in a semirural Ugandan tribe found rates of 10% based on the ICD-8 criteria. Two decades later, a community study by Cooper et al. (1999) in a periurban settlement in South Africa, found rates of 34.7%, an increase of over threefold. Hospital-based studies have found rates of 23% in Goa, India (Patel et al. 2002), 22% in eastern Turkey (Inandi 2002) and 15.8% in Dubai, United Arab Emirates (Goubash and Abou-Saleh 1997). A rural-community study in Rawalpindi, Pakistan, reported over 25% women suffering from depression in the antenatal period and 28% in the postnatal period (Rahman et al. 2007). Over half these women were found to be still depressed a year later (Rahman and Creed 2007). A recent meta-analysis shows that the rates in low- and middle-income countries (LAMIC) are higher than high income countries, ranging from 18–25% (Fisher et al. 2012). Risk factors identified include previous psychiatric problems, life events in the previous year, poor marital relationship, lack of social support, and economic deprivation. Female infant gender was found to be an important determinant of postnatal depression in India, but not in South Africa. Importantly, postnatal depression was found to be associated with high degrees of chronicity, disability and disturbances of mother–infant relationship.

From 1985, when I first met Channi Kumar, he played a seminal role in the research I have conducted with my colleagues on the effects of postnatal depression on child development. At the time I was a very junior researcher, struggling to get my work underway, and his enthusiasm and encouragement were invaluable. Channi played a particularly important role from 1989 when, with the support of the Tedworth Charitable Trust and the Winnicott Trust, the Winnicott Research Unit was established in Cambridge, under the joint Directorship of myself and Peter Cooper, together with Alan Stein. Channi became a key member of the Unit’s advisory committee and, right up until the time of his death, he regularly, and very kindly, provided the wisest counsel and support, and he is still very sorely missed. He was an inspiring friend and colleague, with the deepest compassion and humanity for the plight of the families he helped, and boundless enthusiasm for new perspectives and ideas that could advance understanding and clinical treatment, and I am greatly indebted to him for all his support. This chapter describes research on a prospective longitudinal study of the development of children of depressed and well mothers conducted in the Winnicott Research Unit; much of it took place under Channi’s watch. The work has involved a large number of colleagues, aside from the contributions of Peter Cooper and Alan Stein, and I am particularly grateful to Alison Hipwell, Matt Woolgar, Sheelah Seeley, Janet Edwards, Sarah Halligan, Adriane Arteche, Ian Goodyer, and Joe Herbert for their involvement and support. A diagnosis of ‘postnatal depression’ includes a wide range of possible symptoms, and therefore this unitary term can mask considerable variation in the nature of its presentation. For example, one mother could be slowed down, sleeping excessively, and barely eating, while in another, the episode may manifest itself in restlessness and agitation, with the mother being hardly able to concentrate and feeling constantly irritable. Not surprisingly, then, studies of the effects of postnatal depression on mother-infant interactions have also identified striking variability.

In this chapter we treat plane waves specified by a wave normal and a particle motion vector . Two types of waves, longitudinal waves and shear waves, are observed in solids. For low symmetry directions, there are generally three different waves with the same wave normal, a longitudinal wave and two shear waves. The particle motions in the three waves are perpendicular to one another. Only longitudinal waves are present in liquids because of their inability to support shear stresses. The transverse waves are strongly absorbed. Acoustic wave velocities (v) are controlled by elastic constants (c) and density (ρ). For a stiff ceramic (c ∼ 5 × 1011 N/m2) and density (ρ ∼ 5 g/cm3 = 5000 kg/m3), the wave velocity is about 104 m/s. For low frequency vibrations near 1 kHz the wavelength λ is about 10 m. The shortest wavelengths are around 1 nm and correspond to infrared vibrations of 1013 Hz. Acoustic wave velocities for polycrystalline alkali metals are plotted in Fig. 23.2. Longitudinal waves travel at about twice the speed of transverse shear waves since c11 > c44. Sound is transmitted faster in light metals like Li which have shorter, stronger bonds and lower density than heavy alkali atoms like Cs. The tensor relation between velocity and elastic constants is derived using Newton’s Laws and the differential volume element shown in Fig. 23.3(a). The volume is equal to (δZ1) (δZ2) (δZ3). Acoustic waves are characterized by regions of compression and rarefaction because of the periodic particle displacements associated with the wave. These displacements are caused by the inhomogeneous stresses emanating from the source of the sound. In tensor form the components of the stress gradient are ∂Xij/∂Zk and will include both tensile stress gradients and shear stress gradients, as pictured in Fig. 23.3(b). The force F acting on the volume element is calculated by multiplying the stress components by the area of the faces on which the force acts.

The aim of research is to faithfully observe and describe, predict, determine causation, explain the hitherto unexplained, and conjure further questions. Medical research is concerned with the application of the scientific method to investigating both our population at risk, and our patients. However, it is often impossible to study entire populations. Therefore, many studies are undertaken to analyse representative cohorts before extrapolating the data to the population of interest. Every study is susceptible to bias and error. It is important to determine whether the evidence from an individual or groups of studies is strong or weak. In other words, are these data sets truly representative of the entire population of interest, or are the data distorted due to potential confounding factors? Furthermore, have the investigators presented a measured interpretation of their results in the context of the potential errors within their experimental system? It is these questions that have led to the concept of research evidence, or the hierarchical system in which the strength of the argument within some studies is simply better, or more persuasive, than others. When planning a research study, design is the first consideration. The highest possible level of evidence is a systematic review or meta-analysis of randomized controlled trials (RCTs), or an individual RCT. These are considered the ‘gold standard’ of clinical research. The design of RCTs allows exclusion of confounding factors and bias as much as possible. These studies work very well for certain interventions, such as drug trials. However, where the control and sample groups cannot be blinded, (e.g. ‘sham acupuncture’ or ‘sham manipulation’ as the control), RCTs may be less appropriate. Meta-analyses are considered as type 1 evidence. The more data is pooled, the more valid the results. However, the data may be less relevant to individual patients. Therefore, although potentially the most powerful type of evidence, meta-analyses can have some important limitations. A meta-analysis takes a number of trials from the literature, e.g. 10 trials of 100 patients each.