Types of studies Leili Salehi,PhD Public Health School Alborz University of Medical Sciences 1

Types of Studies A. Observational - no manipulation of study factor by the investigator B. Experimental - study factor is manipulated by the investigator

Descriptive studies  Cannot establish causal relationships  Still play an important role in describing trends and generating hypotheses about novel associations

Good descriptive reporting answers the five basic W questions : Who, what, why, when, where Descriptive studies Case report Case-series reports Who has the disease in question ? What is the condition or disease being studied ? Why did the condition or disease arise ? Where does or does not the disease or condition arise ?

Correlation Study Case Reports Case Series Descriptive studies

Use data from entire populations to compare disease frequencies between different groups. Example: Correlation between per capita daily consumption of meat and rates of colon cancer from a large number of countries. Correlation Study(ecological Study)

Ecological comparison (co relational) 8  Correlation of aggregated or group data  Association on the individual level is unknown and may be different  Many relationships on global level are strictly speaking of ecological nature

Advantages & Disadvantages  Can be done quickly- inexpensively  Often uses already available information  First step in investigating a possible exposure-disease relationship  Inability to link exposure with disease in particular individuals  The lack of ability to control for the effects of potential confounding factors.

Example of an "ecological" comparison: The prevalence of HIV in TB patients (y-axis) against the prevalence of HIV in adults (x-axis). 10

Document unusual medical occurrence Represent the first clues of new disease or adverse effects of exposures Case Reports

گزارش‌هاي موردي (Case Reports) 12 گزارش يك مورد كوري ناشي از بروسلوز

گزارش‌ موارد (Case Series) طغيان تيفوئيد در بين پناهندگان عراقي غرب ايران

Analytic Studies  Attempt to establish a causal link between a predictor/risk factor and an outcome.

Analytical Studies

Research Designs in Analytic Epidemiology همگروهيCohortهمگروهيCohort مقطعي Cross - Sectional مقطعي مورد ـ شاهدي Case - Control مورد ـ شاهدي Case - Control

Population survey Community Survey Epidemiology Survey Since exposure and disease status are assessed at a single point in time Cross Sectional Studies

Cross-Sectional Study در واقع در لحظه‌اي از زمان به بررسي سلامت و بيماري در واقع در لحظه‌اي از زمان به بررسي سلامت و بيماري افراد مي‌پردازيم افراد مي‌پردازيم موارد اندازه‌گيري شده، جزئي از شيوع به حساب مي‌آيد. موارد اندازه‌گيري شده، جزئي از شيوع به حساب مي‌آيد.

 در گروه‌ها و جمعيت‌ها انجام مي شوند  آسان، سريع و كم هزينه هستند  با استفاده از اطلاعات و آمار موجود قابل انجام مي‌باشند  زمينه ساز مطالعات بزركتر هستند.  مثال : بررسی میزان آنمی فقرآهن در دختران دانش آموز شهر کرج

 Impractical for rare diseases  Not a useful type of study for establishing causal relationships  Confounding is difficult to control  Miss diseases still in latent period Disadvantages-Cross Sectional Studies

Cross-sectional study: minuses time - Cannot determine causality USMLE Score Red Bull consumption

Research Question Is the regular consumption of Red Bull associated with improved academic performance among U.S. medical students?

Cross-sectional Study:  Descriptive value:  How many UCSF medical students drink Red Bull?  What is the age and sex distribution of UCSF medical students who drink Red Bull?  Analytic value:  Is there an association between regular Red Bull consumption and test scores among UCSF med students?

 Case Control Studies

25 Case-control studies  Case-Control Studies identify existing disease/s and look back in previous years to identify previous exposures to causal factors.  Cases are those who have a disease.  Controls are those without a disease.  Analyses examine if exposure levels are different between the groups.

Design of Case-Control Studies  Cases: persons/group with a given disease  Controls: persons/group without the given disease  Ascertain exposure or background of the two groups and compare the proportion  Best suited for study of diseases where medical care usually sought, (hip fracture, cancer) because this makes it easier to identify cases

Hospital General Population Case control studies selection of cases

Hospital Friends-neighbors-relatives of cases General Population Controls Age, Sex, Socio-economical Status must be similar with case group Case control studies selection of controls

 Relatively quick and inexpensive compared with other analytic designs  Suitable for long latent periods and rare diseases  Can examine multiple etiologic factors for a single disease. Advantage

Case-control study-minuses  Causality still difficult to establish  Selection bias (appropriate controls)  Caffeine and Pancreatic cancer in the GI clinic  Recall bias: sampling (retrospective)  Abortion and risk of breast cancer in Sweden

 Preliminary results from our cross-sectional and case-control study suggest an association between Red Bull consumption and improved academic performance among medical students  What’s missing? - strengthening evidence for a causal link between Red Bull consumption and academic performance  Use results from our previous studies to apply for funding for a prospective cohort study!

Cohort studies  A cohort study is a comparative, observational study in which subjects are grouped by their exposure status, i.e., whether or not the subject was exposed to a suspected risk factor  The subjects, exposed and unexposed to the risk factor, are followed forward in time to determine if one or more new outcomes occur  The rates of disease incidence among the exposed and unexposed groups are determined and compared.

Cohort Studies  Incidence studies  Longitudinal studies  Follow up Studies  Prospective studies

Cohort Studies Exposed Not exposed Time Disease Present Absent Present Absent

بررسی میزان رخداد سرطان ریه در دو گروه از افرادی که در معدن آهن و معدن روی کار می کنند یا بررسی میزان رخداد سرطان ریه در دو گروه از افرادی که در معدن آهن کار می کنند و افرادی که در اداره کار می کنند Example:

Two types of Cohort studies :  Exposure(+) Disease(?)  Exposure(-) beginning of study Disease(?) Prospective Cohort Study

Two types of Cohort studies :  Exposure(+) Disease(?)  Exposure(-) beginning of study Disease(?) Retrospective Cohort Study

Population People who don’t have disease Exposure(+) Exposure(-) Disease(+) Disease(-) Disease(+) Disease(-) “a” “b” “c” “d”

Cohort Study disadvantages  If incidence rate is low and latent period is long,it is inefficient – large number,more time are needed  If prospective,can be expensive and time consuming  If retrospective require the availability of adequate records  Loses to follow up

Cohort Study Advantages  Exposure-disease relation is very reliable  Can examine multiple effect of a single exposure  Prospective type minimize BIAS  Incidence rate and relative risk can be calculated

30/130 تماس يافتگان سالم و بيمار بيمار سالم تماس نيافتگان سالم و بيمار بيمار سالم بيمار Cohort study بيمار سالم بيمار سالم بيمار جمعيت 200 نفره خوابگاه دختران دانشجو 10/70

Measures of association Disease YesNo Risk Factor YesAB NoCD Risk ratio (relative risk) A A + B C C + D

Oral contraceptives Ovarian cancer

Cohort Study

Ovarian cancer Use of oral contraceptives Control group Problem: adequate control group (matching) and recall bias. Because case- control studies lack denominators, incidence rates, relative risks or attributable risks cannot be calculated. Instead odds ratios are the measures of association.

Comparison of the Characteristics of Cohort Study & Usually very expensive Complete source population denominator experienced tallied Can calculate incidence rates or risks and their differences and ratios Convenient for studying many diseases Case-Control Studies Usually less expensive Sampling from source population Can usually calculate only the ratio of incidence rates or risks Convenient for studying many exposures

selection bias  Is selection bias present ?  In a cohort study, are participants in the exposed and unex-posed groups similar in all important aspects except for the exposure ?  In a case-control study, are cases and controls similar in all important aspects except for the disease in question ?

Interventional study=Experimental study  In an experimental study's, the researcher manipulate at least one independent variable, controls other relevant,and observer the effect on one or more of the dependent variable.  Independent variable is called the experimental variable.

Randomized controlled studies Patients Treatment group Control group Follow up Compare Results Random assignment=Random allocation

Experimental Design Study Population Intervention Control outcome No outcome outcome No outcome

Clinical trials(patient) Field trials(Healthy population) Community trial(Small population) Experimental Study

Clinical Trials New Treatment Comparison treatment Outcome Patients ImprovedNot improved ImprovedN ot improved

Hierarchy of Study Types Descriptive Case report Case series Survey Analytic Observational Cross sectional Case-control Cohort studies Experimental Randomized controlled trials Strength of evidence for causality between a risk factor and outcome

Experimental Studies  Clinical Trials (Involve humans)  Easier to identify (usually explicitly stated in the abstract)  Two main categories of clinical trials: 1. Controlled trials 2. Uncontrolled trials

Clinical Trials

Randomized Controlled Studies

Experimental Studies Controlled trialsUncontrolled trials Self controls Independent concurrent controls External controls RCTNon-randomized

Systematic Reviews &Meta-analyses

Study Pyramid Best Worst

Temporal direction of study designs Lancet 2002; 359: 57-61