Study Design Analytical Observational Studies
Analytical Observational Studies The purpose of an analytic study in epidemiology is to identify and quantify the relationship between an exposure and a health outcome. The hallmark of such a study is the presence of at least two groups, one of which serves as a comparison group.
Analytical Observational Studies Observational studies are those in which individuals are observed and their outcomes are measured by the investigators. Interventional studies are those in which the research subjects are assigned by the investigator to a treatment or other intervention, and their outcomes are measured.
Analytical Observational Studies • Analytical observational studies can be of three types, depending on the time sequence and sampling procedures used to collect data.
Time is Key • The Cohort (Prospective) design measures exposure in the present and the disease in the future. The Case-Control (Retrospective) design measures the disease in the present and looks backwards for exposure history. The simplest is the Cross-Sectional (Prevalence) design which is conducted completely at present. From: Miller School of Medicine, University of Miami http: //hihg. med. miami. edu/code/http/modules/education/Design/Print. asp? Course. Num=4&Lesson. Num=4
Cross-Sectional Study • Cross-Sectional studies can be descriptive or analytical. • In analytical cross-sectional studies, data on the prevalence of both exposure and a health outcome are obtained for the purpose of comparing health outcome differences between exposed and unexposed.
Cross-Sectional Study • Analytical studies attempt to describe the prevalence of, for example, disease or nondisease by first beginning with a population base. • These studies differ from solely descriptive cross-sectional studies in that they compare the proportion of exposed persons who are diseased with the proportion of non-exposed persons who are diseased.
Cross-Sectional Study From: Cross-Sectional Study, http: //www. hsrmethods. org
Case Control Study • Are studies in which patients who already have a specific condition are compared with people who do not have the condition. The researcher looks back to identify factors or exposures that might be associated with the illness.
Case Control Study • They often rely on medical records and patient recall for data collection. These types of studies are often less reliable than randomized controlled trials and cohort studies because showing a statistical relationship does not mean that one factor necessarily caused the other.
Case Control Study • For example, 50 people in Chatham County come to the hospital and are found to have Type 2 Diabetes. 44 of them live in downtown Savannah. You conclude that living in downtown Savannah causes Type 2 Diabetes.
Case-Control Study From: Prehospital Research Support site, http: //prehospitalresearch. eu/? p=1961
Cohort Study • Cohort Studies take a large population and follow patients who have a specific condition or receive a particular treatment over time and compare them with another group that has not been affected by the condition or treatment being studied.
Cohort Study From: Suny Downstate Medical Center, Evidence Based Medicine Tutorial, http: //library. downstate. edu/EBM 2/2400. htm
Prospective Cohort Study • Prospective: none of the subjects have the disease (or other outcome) being measured when the study commences; data analysis happens after a period of time has elapsed. • A prospective cohort study takes a group of similar people (a cohort) and studies them over time. At the time the baseline data is collected, none of the people in the study have the condition of interest.
Prospective Cohort Study • Prospective example: a group of 100 people with high risk factors for AIDS are followed for 20 years to see if they develop the disease. A control group of 100 people who have low risk factors are also followed for comparison.
Retrospective Cohort Study • Retrospective (Historical): the researcher looks at historical data for a group. Some of the people in this group have developed the disease, and some have not. This can result in finding out who has the disease and when they developed it.
Retrospective Cohort Study • Retrospective example: a group of 100 people with AIDS might be asked about their lifestyle choices and medical history in order to study the origins of the disease. A Second group of 100 people without AIDS are also studied and the two groups are compared.
Question • A researcher interested in the relationship between vaccination and autism sends a survey to parents of children who are active patients at a large primary care practice. The survey asks several questions, including whether their children received their childhood vaccines on-time, and whether their children currently have a diagnosis of an autism spectrum disorder. Which of the following correctly identifies the study design used by the researcher? • Case Control Study • Cross Sectional Study • Prospective Cohort Study • Retrospective Cohort Study
Answer • Cross Sectional Study • The researcher used a cross-sectional study design by measuring both the exposure (vaccination) and the outcome (autism spectrum disorder diagnosis) simultaneously.
Question • A study was designed to assess the impact of sun exposure on skin damage in beach volleyball players. During a weekend tournament, players from one team wore waterproof, SPF 35 sunscreen, while players from the other team did not wear any sunscreen. At the end of the volleyball tournament players' skin from both teams was analyzed for texture, sun damage, and burns. Comparisons of skin damage were then made based on the use of sunscreen. The analysis showed a significant difference between the cohorts in terms of the skin damage. Which of the following correctly identifies the study design used by the researcher? • Case Control Study • Cross Sectional Study • Prospective Cohort Study • Retrospective Cohort Study
Answer • Prospective Cohort Study • A group of similar people (volleyball players) were selected. At the time the baseline data is collected, none of the people in the study have the condition of interest. The study measures exposure in the present and the condition in the future.