Developmental psychology is basically the science of studying the psychological changes that occur progressively in people as they age, hence it is also referred to as Human Development. Initially, these studies were done on children in their infancy and in the later stages of childhood. However, later, other phases wherein great changes occur, such as during adolescence, middle age, and old age, were also included, so that these studies now encompass the whole span of life of human beings. These studies include the examination of a wide range of issues, like problem solving capacities, motor skills, acquiring language skills, formation of identity, conceptual understanding, moral understanding, and other psycho-physiological factors. The scientists involved in development psychology analyze key issues like whether children differ from adults qualitatively or merely lack the ability of adults to draw upon experience. Some of the other questions they deal with are whether human beings develop by gradually accumulating knowledge or by transitioning from stage to stage of thinking; or whether children have innate knowledge when they are born or comprehend things about life through experience; and if development is determined by social circumstances or due to something existing innately in each child. There are many types of research methods used in the study of developmental psychology, of which the two main ones are longitudinal and cross-sectional research. Scientists usually use both the methods to find out how development takes place throughout the whole lifespan of individuals. Longitudinal Research Method: This type pertains to the study of a group of people over an extensive span of time. Data is collected at the commencement of the study, and then additional data may be gathered from time to time, all through the length of the research. It can sometimes last many decades. This type is useful, because it allows scientists to study the changes that occur over time. However, the drawback of this type is that it involves huge amounts of time and can often be very expensive. As a result, these studies are usually conducted on relatively small groups of individuals, which means that it is usually difficult to relate the results to larger populations. Another problem that often occurs is that participants quit from the study, which shrinks the size of the sample even further, thus reducing the amount of data even further. There are two main types of longitudinal research studies: panel studies and cohort studies. In panel studies, a cross-section of people are surveyed at regular intervals. In cohort studies, a cohort, which means a group that experiences the same event (usually birth) at a particular period of time, is selected as a sample for studies. Another is known as retrospective study, which involves looking back over a period of time. For example, the medical records of past years may be studied to establish the existence of a trend. Cross-sectional Research Method: In this method, different groups of individuals belonging to different ages are studied. For instance, a group comprising young adults may be observed, and the data collected from the observation may be used to compare another gathered from a group made up of elderly subjects. One of the benefits of this kind is that it can be carried out fairly quickly, since the data of the research is collected at a single point of time. However, as the data collected is from generations of individuals who have similar cultural experiences, these may play a part in the process of development. This results in making it difficult to ascertain whether experience or the process of aging is the causative factor. As is the case with most other types of research methods, developmental psychology research is based on hypothesis. A hypothesis is a statement that can be tested and which demonstrates what the scientist anticipates to find in a study. The aim of research in developmental psychology is, thus, to gather data and information about an issue, analyze it, derive conclusions on the basis of the data, which then can be added to previous theories of development that exist in order to expand knowledge and understanding in this field.