1

Different Types of Epidemiologic Studies

Kamran Yazdani, MD MPHDepartment of Epidemiology and Biostatistics

School of Public Health

2

CASP

Critical Appraisal Skills Programme

http://www.phru.nhs.uk/Pages/PHD/resources.htm(Appraisal Tools)

3

CASP• Major Points:

• Is the Objective CLEAR?• Is the Method (Design & Analysis)

APPROPRIATE?• Dealing with:

• Selection Bias• Information Bias• Confounding• Chance error

• Reporting (Analysis, Results)• Interpreting (Association, Causation)• Generalizability• Consistency, Coherence

SC

RE

EN

ING

4

STrengthening the Reporting of

OBservational studies in Epidemiology

http://www.strobe-statement.org/Checklist.html(Checklists)

STROBE

5

Epidemiology

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.

6

Aim of epidemiological studies

•To determine distribution of disease

Descriptive Studies

•To examine determinants of a disease

To judge whether a given exposure causes or prevents disease

Analytical Studies

7

انواع مطالعات

با هدف بررسي و توصيف يك توصيفي:

وضعيت بدون آنكه قصد بررسي يك رابطه

)آزمون فرضيه( را داشته باشيم

با هدف بررسي يك رابطه ، اختالف يا تحليلي:

ارتباط )آزمون فرضيه( صورت مي پذيرد

8

Epidemiologic Design Strategies•Descriptive studies

•case reports, case-series, cross-sectional

•Analytical studies–Observational studies

•Cross-sectional•Case-control studies•Cohort studies

–Intervention studies •Clinical trials•Field trials•Community trials•Experimental (animals)

•Diagnostic Tests studies or Process Research

9

انواع مطالعات

توصيفي تحليلي

مشاهده اياي مداخله

كارآزمايي باليني

كارآزمايي اجتماعي

كارآزمايي ميداني

مقطعي

مورد شاهدي

كوهورت

اكولوژيك

گزارش مورد

گزارش موارد

10

Advantages of Case Reports and Case series

•Allows for the description of new disease processes.

•Allows for the description of outcomes associated with rare diseases or rare features of any disease.

•To formulate hypotheses of the association

11

Disadvantages/Limitations of Case Report & Case Series

•Impossible to determine disease frequency.

•Cannot establish causality between exposures or risk factors and disease outcome.

12

Case Report example

In 1961, a published case report of a 40 year-old woman who developed pulmonary embolism after beginning use oral contraceptive

13

Case Series example

In Los Angeles, five young homosexuals men, previously healthy, were diagnosed with pneumocyst cariini pneumonia in a 6-month period (80-81)

14

Case Series example

RESULTS: Twelve patients with histopathologically confirmed tumours detected after extraction of teeth were studied. There were 11 males and one female giving a male to female ratio of 11:1. They ranged in age from 15-85 years with a mean age of 53 years. Pain and swelling were the most common presenting complaints. The mandible was more often involved seven (58.3%) cases while five (41.7%) cases occurred in the maxilla. Squamous cell carcinoma (in 9 cases) was the most common malignant neoplasm among these patients.

15

•Cross-Sectional Studies measure existing disease and current exposure levels.

•They provide some indication of the relationship between the disease and exposure or non-exposure

Cross-sectional studies

16

Cross Sectional Studies (contd)

•Sample without knowledge of Exposure or Disease

•Sample at one point in time

•Mostly prevalence studies/surveys

17

•Good design for hypothesis generation

•Can estimate overall and specific disease

prevalence and sometimes rates

•Can estimate exposure proportions in the

population

• Can study multiple exposures or multiple

outcomes or diseases

Cross Sectional Studies(Advantages)

18

•Relatively easy, quick and inexpensive!!!

•Best suited to studying permanent factors (breed,

sex, blood-type) to deal with TEMPORALITY.

•Often good first step for new study issue

Cross Sectional Studies(Advantages)

19

• Impractical for rare diseases• Not a useful type of study for establishing

causal relationships• Confounding is difficult to control• Problems with temporal sequence of data• hard to decide when disease was actually

acquired• miss diseases still in latent period• recall of previous exposure may be faulty

Cross Sectional Studies(Disadvantages)

20

dc

ba

Yes No

Disease Status

Yes

No

Exposure Status

a +b

c +d

b +da +c N

Total

Cross Sectional Studies

21

7514

873

Yes No

Depression

Yes

Nolow SES

90

89

16217 179

Total

Cross Sectional Studies

22

Cross Sectional Studies

• Points need concern:

• Sampling• Protocol (Quality Assurance & Control)• Appropriate Analysis• Appropriate and Fair Interpretation

23

Randomized Clinical Trials

New Treatment

Comparison treatment

Outcome

Participants

(Randomization)

Improved Not improved

Improved Not improved

Population

Inclu & Exclu

24

7525

8713

Yes No

Cure

A

B

Treatment100

100

16238 200

Total

Randomized Clinical Trials

Adobe Acrobat 7.0 Document

25

Cohort / Follow-up Studies

Study population(Non-diseased)

Exposed

Non-exposed

Disease +

Disease +

Disease -

Disease -

26

Cohort study, at a glanceCase control Cohort

Study group Diseased/ healthy

Exposed/ unexposed

Measure of effect

OR, AR Risk, RR,OR.AR

temporal Hard to establish Easy to establish

multiple exposures outcomes

time Short Long

cost inexpensive Expensive

Population size

small Large

Information bias

exposure Outcome

Best when D rare E frequent E rare D frequent

Problems Control selection

Exposure information

Unexposed selection

change over time

27

جدول توافقي در مطالعه ي كوهورت

dc

ba

Yes No

Disease Status

Yes

No

Exposure Status

a +b

c +d

b +da +c

Total

n1

n2

N

28

آيا مطالعه كوهورت هميشه تحليلي است؟

29

مواجهه پي آمد

مواجهه پي آمد

مواجهه پي آمد

زمان حال

مطالعه كوهورت ـ انواع

30

Prospective vs. retrospective Cohort Studies

Prospective Cohort Studies– Time consuming, expensive– More valid information on exposure– Measurements on potential confounders

Retrospective Cohort Studies– Quick, cheap– Appropriate to examine outcome with long latency

periods– Admission to exposure data– Difficult to obtain information of exposure– Risk of confounding

31

مطالعه كوهورت چه موقع؟مناسب تر است

شواهدي موجود باشد••LOSS TO FOLLOW UP را بتوانيم كنترل

كنيممدت پي گيري نسبتا5 كوتاه باشد•بتوانيم كوهورت تاريخي انجام دهيم•مواجهه نادر باشد•

32

Selection of the Exposed Population

Sample of the general population:Geographically area, special age groups, birth cohorts (Framingham Study)

A group that is easy to identify:Nurses health study

Special population (often occupational epidemiology):

Rare and special exposure

33

Selection of the Comparison Population

• Internal Control Group– Exposed and non-exposed in the same Study

population (Framingham study, Nurses health study)

• Minimise the differences between exposed and non-exposed

• External Control Group– Chosen in another group, another cohort

(Occupational epidemiology: Asbestosis vs. cotton workers)

• The General Population

34

Bias• Selection bias:

– Non-representativeness (unequal in e+ & e-)– Non-response during data collection– Losses to follow up– Healthy worker effect

• Information bias– Misclassification on exposure– Misclassification on event

35

CASP questions

Adobe Acrobat 7.0 Document

36

Case-control study

Study Population

Cases

Controls

Exposed

Non-exposed

Exposed

Non-exposed

37

Direction

exposure outcome

38

Applications

• Diseases with long latency period

• For best use of time and money

• The best for rare diseases & useful for prevalent diseases

• Mutiple exposures

39

Case-control study, at a glanceCase control Cohort

Study group Diseased/ healthy

Exposed/ unexposed

Measure of effect

OR, AR Risk, RR,OR.AR

temporal Hard to establish Easy to establish

multiple exposures outcomes

time Short Long

cost inexpensive Expensive

Population size

small Large

Information bias

exposure Outcome

Best when D rare E frequent E rare D frequent

Problems Control selection

Exposure information

Unexposed selection

change over time

40

BIASes

• Selection

• Information

41

Selection of cases

• Case definition is more important than other studies:

– Strict diagnostic criteria (high degree of caseness)

– Homogenous diseases (one well-defined outcome or health-related state)

42

Selection of cases

• Sources of cases– Population– Hospital (available, better dx, low inf bias,

high sel bias)– Registry– …

• Are the cases representative of total population or a fraction of it?

43

Selection of cases

• Incident vs. Prevalent– Selection & Information BIASes

44

Selection of controls

• Study base– Sel bias

• Deconfounding– confounding

• Comparable accuracy– Info bias

45

Selection of controls

سوال:•آيا مي توان براي يك گروه بيمار كه از –

بيمارستان انتخاب شده اند، گروه كنترل را از جامعه گرفت؟

46

Types of controls

• Population controls– Friend, RDD, neighbor controls

• Hospital controls– Similar disease as controls

47

Hospital controls

• Similar study base

• Similar quality of information

• Convenience

48

Population controls

• Tax lists, vote lists, telephone directories,…

• RDD: – Selection bias due to higher ses., families with

more than phone lines and family size

49

Matching

50

Ratio of controls to cases

• Statistical considerations (increasing power) – Unsuitable for very low or very high powers

under equal sizes

51

Ratio of controls to cases

• Control to case ratio: up to 4-fold:

case cont

• 1:1 200 200

• 1:2 150 300

• 1:3 133 400

• 1:4 125 500

• 1:5 120 600

52

جدول توافقي در مطالعه ي

مورد ـ شاهدي

dc

ba

Yes No

Disease Status

Yes

No

Exposure Status

a +b

c +d

b +da +c

Total

n1 n2 N

53

CASP questions

Adobe Acrobat 7.0 Document

54

OR و RRمقايسه

مي تواند برآورد ORدر مطالعه كوهورت • باشد اگر:RRخوبي از

بيماري بروز بااليي نداشته باشد• مي تواند ORدر مطالعه مورد شاهدي •

باشد اگر:RRبرآورد خوبي از بيماري بروز بااليي نداشته باشد•موردها نماينده بيماران جامعه باشند•شاهدها نماينده سالمهاي جامعه باشند•

55

Comparing cohort and case controlCase control cohort

Study group Diseased/ healthy

Exposed/ unexposed

Measure of effect

OR, AR Risk, RR,OR.AR

temporal Hard to establish Easy to establish

multiple exposures outcomes

time Short Long

cost inexpensive Expensive

Population size

small Large

Information bias

exposure Outcome

Best when D rare E frequent E rare D frequent

Problems Control selection

Exposure information

Unexposed selection

change over time

56

Intuition and Logic in Research

Dominant Mental ActivityIntuition

Feeling

Judgement

Experience

Analysis

Experiment

Control over variance

Hi

Potential for Misinterpretation

Qualitative

Research

Case Report

Case Series

Cross-sectional Study

Case-control Study

Cohort Study

Clinical trials

Lo

LoHi

57