Basic Epidemiology Teaching and Learning Pack

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Title of Module Basic Epidemiology,Type of resource Tutor s Notes. Author Sarah Head,Subject expert Paul Nelson,Date last updated 12 10 08 By Helen Barratt. These resources are freely available to be copied and used for teaching and public health studies Please. acknowledge author and LTPHN for publication LTPHN 2008. LTPHN 2008 Original Author Sarah Head,Last Updated 12 10 08. EPIDEMIOLOGY TUTOR S NOTES,What will this module cover. Measurement disease frequency and measures of effect. Association causation and the role of chance bias and confounding. Study design, Epidemiology is the study of the distribution and determinants of health related states or.
events in specified populations and the application of this study to control of health. problems 1, More simply it has been described as the study of patterns of disease occurrence in human. populations and the factors that influence these patterns 2. The word epidemiology is derived from Greek word Epidemos Epi meaning upon. and demos meaning the people It was used by Hippocrates to describe disease states that. visit the community, The word was first used in the English language in the 1850s and originally referred to. infectious disease outbreaks In contrast it now refers to studying patterns of all disease. within the population both infectious and non infectious. Concepts involved, Epidemiology examines how disease and determinants of disease are distributed at the. population level 3 It is based upon two basic assumptions that. a Human disease does not occur randomly, b Human disease has causative and preventative factors which can be identified by. examining how these factors vary in time person and place within a population. Epidemiologists are interested in examining those without disease in order to compare them. to those with disease Done systematically this leads to findings of what factors e g in. time person place are associated with a population developing the disease or remaining. healthy In turn this allows us to do put it to two main uses. a Public health considering the knowledge supplied by epidemiology to provide. interventions at a population based level that will improve the overall health of the. population, Last J A Dictionary of Epidemiology New York Oxford University Press 2001.
Lillienfeld A Lillienfeld D Foundations of Epidemiology New York Oxford University Press 1980. Hennekens CH Buring JE Epidemiology in medicine Boston Little Brown and Company. LTPHN 2008 Original Author Sarah Head,Last Updated 12 10 08. b Clinical work using the knowledge supplied by epidemiology about risk factors for. disease health and related treatments to provide patients with an effective. intervention in order to treat and prevent disease. In essence epidemiology is based upon three basic components to explore patterns of disease. occurrence,Disease distribution in space i e where. Disease distribution in time i e when how often, Disease determinants i e what is associated socioeconomic lifestyle drugs etc. What can epidemiology be used for, Understanding the natural history of a disease for example epidemiological studies of. hepatitis C have helped us understand that initial infection is generally asymptomatic and. approximately 80 85 of patients are left with chronic infection and may go on to. develop cirrhosis, To describe patterns of disease for example concerning sources of HIV transmission.
epidemiological techniques that have demonstrated that heterosexual transmission has. become an increasingly important means of transmission in comparison to homosexual. transmission and infected needles, Identify the cause of a new syndrome or even a new risk factor for a well described. disease For example when the first cases of Severe Acute Respiratory Syndrome were. recognised even though it was laboratory techniques that eventually identified the. organism it was epidemiological techniques that provided information about the likely. modes of transmission risk factors natural history etc of the disease. Assess the risks associated with a harmful exposure. Evaluation of a treatment intervention for example epidemiological techniques are. used in trialling the effectiveness of a vaccine in preventing disease in a population. Identifying health service use needs trends for example epidemiological studies of a. population form an essential part of a needs assessment in order to estimate levels of. Why is epidemiology important, In addition to the above there are the following issues. Only by examining disease at the population level is it possible to truly identify an. association For example not until the evidence at a population level was examined. by Doll was it possible to make a substantial case that smoking caused ill health. Without epidemiology all we have are a collection of anecdotal reports. History has demonstrated that epidemiological techniques often uncover associations. between determinants of disease and ill health before they can be demonstrated in. laboratory studies For example epidemiology revealed the association between prone. sleeping position and Sudden Infant Death Syndrome SIDS which in turn led to a. change in health advice and decreased SIDS rates, Epidemiology helps us understand population health needs and is therefore important. in planning health policy For example the changing epidemiology of TB in the UK. LTPHN 2008 Original Author Sarah Head,Last Updated 12 10 08. has resulted in the removal of the universal immunisation programme and instead. services now focus on high risk populations only, Understanding epidemiological techniques also allows us to appreciate the wider.
determinants of health Epidemiological studies have shown clear socio economic. gradients in the uptake of breast cancer screening women of lower socio economic. status are less likely to access screening In addition to this a recent study on breast. cancer found that women living in deprived areas are more likely to have high. grade advanced stage breast cancer at diagnosis than women in more affluent areas. Definition, Any trend in the collection analysis interpretation publication or review of data that can. lead to conclusions that are systematically different from the truth Lasts Dictionary of. Epidemiology, Can occur to all types of study though different study designs are more less at risk of. particular types of bias, Although difficult to avoid totally can be minimised with careful planning of how. study will be carried out, Number of different ways of classifying bias but one of the best established is. considering it as 2 main types,o Selection bias,o Information bias.
Bias is a type of systematic error which unlike confounding cannot be adjusted for. after the study has been carried out and the data has been collected. Selection Bias, This is bias that occurs as a result of errors during selection of the study population. Consequently the relationship between the exposure and disease differs between those who. were eligible to be enrolled in the study and those who were actually enrolled. Selection bias can occur when, a Differential surveillance diagnosis referral of patients into the study. Cases may be more less likely to be selected according to their exposure status For. example when a link was first made between the Oral Contraceptive Pill OCP and. thromboembolism women presenting with symptoms of thromboembolism were. more likely to be investigated if they were known to be on the OCP Therefore a. case control study using cases extracted from hospital records would demonstrate an. exaggerated exposure to the OCP in relation to thromboembolism. Cases and controls sharing risk factors that may be related to developing outcome For. example a case control study of oral cancer and drinking If the controls are drawn. from a hospital population then they may be more likely drink than the general. population and thus share risk factors for oral cancer This would result in a decrease. in the apparent risk of exposure on developing disease. Biased recruitment of patients into the study population For example a GP asked to. identify patients for a study on counselling and depression may only refer those. LTPHN 2008 Original Author Sarah Head,Last Updated 12 10 08. patients with milder disease who he feels will respond well to the treatment This may. lead to an exaggerated treatment effect of counselling. b Refusal Non response from cases or controls, Attempting to access controls by calling them at home during office hours will result. in a control group with high levels of non working individuals whose age profiles. may be older This would not be a suitable control group if cases are mainly young. working adults, Consider a case control study on possible links between obesity and physical activity.
cases of obesity may refuse to be involved if they feel stigmatised. c Completeness of follow up, This is particularly relevant to cohort studies where individuals may be followed over. many years and subjects can consequently be lost to follow up Although this may just. occur by chance it can also be related to the exposure or the outcome If this varies. between groups then the results may be biased For example in a cohort study. investigating the long term effects of an occupational exposure subjects who become. unwell possibly as a result of the exposure of interest may leave their job and. become lost to follow up This would give a biased estimate of the effect of the. Dealing with selection bias, This varies depending on the individual situation but in general for methods to minimise. selection bias in a case control study include, Clearly defining the population of interest so that cases and controls can be extracted. from exactly the same population, Selecting controls from more than one source e g hospital and community There. are pros and cons to using hospital controls vs neighbourhood controls so using both. methods may compensate for weaknesses of one source. If there is concern about possible exposure as a source of bias then additional. information can be collected about this and the results analysed in separate strata. Selection bias is a particular a problem for case control studies because exposure and. outcome have already occurred when the data is collected In contrast in cohort. studies RCTs the outcome has not already occurred at the time of measurement of exposure. status and therefore knowledge of the patient s outcome status cannot influence their. enrolment in the study,Information Bias, Information bias occurs when there is misclassification of the disease and or the.
exposure status This can occur in the following ways. a Recall Bias, In case control studies in particular where exposure status may be measured by an. interview questionnaire completed by the participant. Participants recall of their exposure may be dependent on their disease status cases. are often likely to overestimate exposures in comparison to controls For example in. a case control study about congenital malformation and the use of medication during. pregnancy the mother of a case is more likely to remember any drugs she took in. pregnancy than the mother of a control, In addition people may under report on sensitive issues e g history of STD. Use of proxies to gather information e g interview with next of kin may also result. LTPHN 2008 Original Author Sarah Head,Last Updated 12 10 08. Additional sources of information such as prescriptions and patient records can be used to. minimise recall bias,b Observer Interviewer bias, This occurs when those gathering information about the subjects introduce bias by the. way they record interpret the information, Knowledge of exposure status may influence classification of disease status and.
knowledge of disease status may conversely influence classification of exposure. status For example an interviewer collecting information for a case control study. about passive smoking and lung cancer may record a case as having a higher exposure. status than a control, Observer bias does not only occur in interviews It can also occur in situations such as. taking measurements or extracting data from patient records. Methods of minimising observer bias include blinding those collecting information to the. status of participants using rigorous study protocols and training of those collecting. information Additionally using automated procedures such as an electronic blood pressure. monitor rather than sphygnomometer as well as validation of study techniques by external. personnel may also help,LTPHN 2008 Original Author Sarah Head. Last Updated 12 10 08,Association and Causation, In epidemiology we look for associations between exposure and outcome e g inadequate. folic acid in diet and neural tube defects However just because an apparent association has. been found between exposure and outcome is not sufficient for implying causation Firstly. not all associations will be true associations and secondly association does not necessarily. imply causation For example a study that demonstrates an apparent association between. hypertension in pregnancy and low birthweight children may be. a Only an apparent association and in fact the result of systematic or random error. b A true association that occurs due to chance rather than an exposure causing an. Basic Epidemiology Teaching and Learning Pack Contents Title Type of resource Introduction Tutor notes Tutorial 1 Epidemiology examines how disease and determinants of disease are distributed at the population level 3 It is based upon two basic assumptions that a Human disease does not occur randomly b Human disease has causative and preventative factors which can be identified by

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