Introduction to Study designs

DMALIMUDepartment of Epidemiology and Biostatistics, MUHAS.

TSHS 500/=

Case-control

Cohort

Individuals

InterventionRetrospective

Prospective

Descriptive

Populations

Analytical

Observational

Case-series

Cross-sectional

Correlational/ecological

Clinical trials

Epidemiological studies

Why do epidemiologic studies?

How big is the problem?

Are two factors related, (cause and effect)?

FELP

Introduction to Descriptive studies

Case-control

Cohort

Individuals

InterventionRetrospective

Prospective

Descriptive

Populations

Analytical

Observational

Case-series

Cross-sectional

Correlational/ecological

Clinical trials

Epidemiological studies

Learning ObjectivesWhen you have completed this session you will be

able to: Describe the differences between and descriptive

and analytic studies Describe the differences between a case report and

a case series Describe the characteristics of an ecological study Describe a cross sectional study and explain its

advantages and disadvantages Explain the uses of the descriptive study types

Descriptive versus Analytical epidemiology

Descriptive epidemiology:

• generates idea(s) or hypothesis for associations between risk factor and illness

Analytical epidemiology:

• uses a comparison group to establish an association between risk factors and illness in the two groups

Descriptive Studies

The most frequent design strategy found in the epidemiologic literature

Used to describe the distribution of disease by time, place, person and assoc. factors

Describing these factors does not link them However we can identify unusual

distributions or correlations Useful for Hypothesis generation and health

planning

Using Descriptive Studies for Hypothesis Formulation Person – “Who is getting the disease?”

Age, race, sex Place – “Where are the rates of disease

highest and lowest?” Time – “ When does the disease occur

commonly or rarely?” and “ Is the frequency of the disease now different from the corresponding frequency in the past?”

PersonPlace

Time

Cases

0

5

10

15

20

25

1 2 3 4 5 6 7 8 9 10

0200400600800

10001200

0-4 '5-14 '15-44

'45-64

'64+

Age Group

Descriptive Epidemiology

Who? Where? When?

Categories of Descriptive Studies

Populations (correlational or ecological studies)

Individuals Case reports Case series Cross-sectional surveys

Correlational or Ecological Studies

Based on aggregate measures of exposure and outcome from several populations.

The population is the unit of observation available for study.

Exposures:- What percent of a population smokes?- What percent of 1-year old children are vaccinated against measles?- What percent of a population has piped water?

Correlational or Ecological Studies

Outcomes:- What percentage of a population died from MI?- What percentage of children had measles last year?- What percentage of population had episodes of diarrhea?

Correlational or Ecological Studies

Advantages-Easy to do

-Use available data (“administrative” or other aggregate data)

-Can be done in population with widely differing characteistics

-Generate hypotheses for additional study

Correlational or Ecological StudiesDisadvantages

-Unable to examine data for individuals; data on exposure and data on outcome are collected independently

-No assurance that persons with exposure (risk factor) of interest are the same ones with the outcome (disease) of interest

-Association at the aggregate level may not reflect association at the individual level - the ecologic fallacy

-Unable to adjust for potential confounding factors.-Poor correlation doesn’t mean no association

Descriptive Studies

Populations (correlational or ecological studies)

Individuals Case reports Case series Cross-sectional surveys

Case reports

The individual is the unit of observation available for study.

Clinical case with “unusual” clinical picture

May suggest an etiological association

Case series

First case report may stimulate compilation of additional case reports….a case series

(e.g. occurrence of Pneumocystis carinii among a group of young, homosexual men with no history of immune deficiency)

Case reports or Case series

Advantages:Use available clinical dataDetailed individual dataSuggest need for investigation (hypothesis generation)

Disadvantages:May reflect experience of one person or one

clinicianNo explicit comparison group

Descriptive Studies

Populations (correlational or ecological studies)

Individuals Case reports Case series Cross-sectional surveys

Design of cross-sectional study

Defined population

Exposed: Have disease

Exposed; Do not have disease

Not exposed; Have disease

Not exposed: Do not have disease

Gather data on exposure and disease

Cross Sectional Study

ExposureOccurrence ?

Time of studyDiseaseOccurrence ?

+-

+ -ill

exp

Selection of population

Cross-sectional study Also known as “prevalence” studyDesign

Identify research questionSpecify target and accessible populationSample the populationMeasure variables of interest (usually a survey)

Thus: participants classified by exposure and disease status at the same time. This allows identification of prevalent cases, calculation of prevalence rates.

Cross-sectional surveys

Measure variable(s) at a single time: prevalence studies (“snapshot”) useful for events/diseases when

chronic common non-fatal

temporal relationship cannot be established unless exposure permanent

If exposure unalterable, cross-sectional survey analytical study

Cross-sectional study Strengths/Advantages

Can study entire populations or a representative sample

Provide estimates of prevalence of all factors measured

Standardized data collection tool. May be quick and inexpensive Valuable in assessing health status and health

care needs of a population Can be repeated to get trend data Help in hypothesis generation

Cross-sectional study Weaknesses/disadvantages

Information on disease and exposure collected simultaneously, therefore difficulty establishing that cause antedated effect.

Use of prevalent cases means data reflects determinants of survival as well as etiology

Cases may be misclassified due to changes in exposure or poor memory of earlier exposures

Not good for rare diseases or exposures Cannot measure risk Cant study temporal relationship

Data analysis and interpretation of descriptive studies Cross-sectional studies and surveys are

measuring prevalence Well-suited for describing variables and their

distributions – Eg. Kenya Demographic and Health Survey

Design of a cross-sectional study

Disease No Disease

Job A

Job B

a b

c d

Prevalence of disease in exposed (Job A) = a/a+b

Prevalence of disease in unexposed (Job B) = c/c+d

Presentation of Cross Sectional Data2x2 table

Exposed

Not exposed

ill not ill

a

c

b

dPrevalence in exposed (Pe+) = a/(a+b) Prevalence in non-exposed (Pe-) = c/(c+d)Prevalence ratio = a/(a+b) / c/(c+d)

Job A (hazardous)

80 healthy 80 well

100 workers

20 resp 10 ill symptoms

Job B (non- hazardous)

95 healthy 80 well

100 workers

5 ill 10 ill

Prevalence ill job A: 20/100 = 20%Prevalence ill job B: 5/100 = 4%Prevalence ratio: 5

Cross-sectional surveys

Association Measures in Cross Sectional Studies

Example: Corporal hygiene and trachoma

Poor Hygiene Trachoma Healthy TotalYes 54 337 391

No 50 1 459 1 509

Total 104 1 796 1 900

Prevalence ratio = 13.8 / 3.3 = 4.2

Prevalence13.8 %

3.3 %

% exp 51.9% 18.8%

RecapNow that you have completed this session you will be

able to: Describe the differences between and descriptive

and analytic studies Describe the differences between a case report and

a case series Describe the characteristics of an ecological study Describe a cross sectional study and explain its

advantages and disadvantages Explain the uses of the descriptive study types

What is the prevalence of trachoma, and is it associated with poor hygiene? Population of 1900

Poor hygiene TrachomaYes No

Yes 54 337No 50 1459

Exercise

Calculate Prevalence of Chlamydia in this population of STI patients.

Calculate prevalence ratio for Chlamydia among OCP users vs. non-users.