Descriptive study design

Overview of epidemiologic design strategies I. Observational Descriptive Populations{Correlational studies} Individual Case report Case series Cross sectional studies Analytic studies Case control Cohort Retrospective Prospective II. Interventional/Experimental Randomized controlled trial Field trial Clinical trial

Descriptive Studies Relatively inexpensive and less time-consuming than analytic studies, they describe, Patterns of disease occurrence, in terms of, Who gets sick and/or who does not Where rates are highest and lowest Temporal patterns of disease Data provided are useful for, Public health administrators (for allocation of resources) Epidemiologists (first step in risk factor determination)

cases with denominator One case of unusual findings Case Report Multiple cases of findings Case Series Descriptive study designs include case reports, case series, incidence studies, and ecologic studies. The case report is the most elementary study design in the literature. It generally describes an injury or injuries to one or two individuals that have been identified in a medical setting. There is also usually a unique feature to the noted chronic disease . The case series design is an extension of the case report. In a case series, a number of events are described. These events usually have been observed over a set period of time (such as one year) and are identified from one reporting source (e.g. a hospital). The descriptive epidemiology study is noted by the collection of events over a defined population base and by the use of denominator data to determine rates. The most frequent information generated from these designs are incidence rates for injuries. The ecologic study is a hypothesis generating study. Usually using group-level data, it examines if two factors are correlated with each other. Population-based cases with denominator Cross sectional

Case report/Case series Report of a single individual or a group of individuals with the same diagnosis Important interface between clinical medicine & epidemiology Most common type of studies published in medical journals{1/3rd of all} AIDS ~ b/w oct1980-may81, 5 cases of P.carinii pneumonia were diagnosed among previously healthy young homosexual males in L.A.

Case Reports Detailed presentation of a single case or handful of cases Generally report a new or unique finding e.g. previous undescribed disease e.g. unexpected link between diseases e.g. unexpected new therapeutic effect e.g. adverse events Case reports are in many ways “sentinel events” which can lead to testable hypotheses

Case Series Experience of a group of patients with a similar diagnosis Assesses prevalent disease Cases may be identified from a single or multiple sources Generally report on new/unique condition May be only realistic design for rare disorders Case series also provide suggestive evidence many times leading to more extensive testing.

Case Series Advantages Disadvantages Useful for hypothesis generation We can aggregate cases from disparate sources to generate hypotheses and describe new syndromes Example: hepatitis, AIDS Informative for very rare disease with few established risk factors Disadvantages Cannot study cause and effect relationships because there is no relevant comparison group Cannot assess disease frequency Case series From Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Case_series

Cross sectional study a cross-sectional study (also known as prevalence study) is a type of observational study that involves the analysis of data collected from a population, at one specific point in time. Cross-sectional studies are most familiar to us as surveys

An “observational” design that surveys exposures and disease status at a single point in time (simultaneously) in a defined population) Cross section al studies are some of the first studies completed because of ease and low cost time Study only exists at this point in time Cross sectional

It a “snapshot” or “still life portrait”. e.g. - cholesterol measurement and ECG measured at same time Determines prevalence at a point in time It can be used to measure any factor that can be reported by respondents or assayed noninvasively and that does not require follow-up to assess. Cross-sectional studies are the source of most of what we know about the population other than vital statistics. Cross sectional

A type of cross-sectional study, not usually described as such, is a population census, where an entire population is enumerated. A census provides denominator data for many purposes, such as estimation of birth and mortality rates, assessing the generalizability of studies conducted in subpopulations, and projecting the results from smaller studies. Population censuses involve a huge effort. People can be difficult to find and to count, and may not want to provide data.

Relatively cheap, easy and quick. Advantage Relatively cheap, easy and quick. It assesses the prevalence of several diseases / exposures in the population It provides clues for further research into the etiology of diseases (identifying associations) Can be useful for comparison of different groups of people with certain disease among countries or different geographical areas Provide data for health planning and priority setting of health problems Also called Surveys Prevalence Studies Cross sectional

Disadvantages Cannot establish temporal relationship between exposure and disease Not suitable to study rare disease or disease with short duration It considers prevalent rather than incident cases, results will be influenced by survival factors Cross sectional

Ecological (correlation) study An observational study in which at least one variable is measured at the group level. It is appropriate for initial investigation of causal hypothesis. Measurements made at population rather than individual level

The x-axis indicates per capita sugar calories consumed per day. The y-axis represent the age-adjusted mortality rate for prostate cancer. Each dot represents a country, plotting the per-capita sugar consumption and the age-adjusted prostate cancer mortality rate for that particular country. Correlation

Per-capita sugar consumption is an ecological variable because it is the average measure of exposure to sugar for all the people in the country. It does not mean that every person in the country ate exactly the same amount of sugar. Similarly, the mortality rate is a group-level variable because it represents the country's experience, not any individual person's experience in the country. Do the data support the hypothesis that increased sugar consumption is associated with increased prostate cancer mortality? - YES! Do the data determine that sugar consumption causes prostate cancer death? NO! Correlation

Correlational Studies (Ecological Studies) ADVANTAGES Weaker inference, but easier to gather data We can generate hypotheses for case-control studies and environmental studies We can target high-risk populations, time-periods, or geographic regions for future studies

Correlational Studies LIMITATIONS Because data are for groups, we cannot link disease and exposure in individual We cannot control for potential confounders Data represent average exposures rather than individual exposures, so we cannot determine a dose- response relationship Caution must be taken to avoid drawing inappropriate conclusions, or ecological fallacy Correlation

Ecological fallacy “The bias that may occur because an association observed between variables on aggregate levels does no necessarily represent the association that exists at the individual level.” Correlation