cross sectinal and methods

7/31/2019 Cross Sectinal and Methods

1/27

Observational Research

Methods

Ivan J Perry

Department of Epidemiology &

Public Health

University College Cork


2/27

Scenario

It is suggested that occupational exposure to

volatile anaesthetic agents causes depression.

You wish to test this hypothesis.


3/27

At the end of this lecture you should be able to:

List and distinguish between the four major types of

observational analytical study designs, including the

strengths and limitations of each design

Understand the difference between prevalence and incidence

Given a research question you should be able to select the

appropriate study design

Learning objectives


4/27

Observational

Descriptive Analytical

Experimental


5/27

Type of

Study

Alternative

Name

Unit of

Study

Ecological

Cross-sectional

Case-control

Cohort

Correlational

Prevalence

Case-reference

Follow-up

Populations

Individuals

Individuals

Individuals


6/27

Ecological studies often provide the first tentative evidence ofan association between a causal factor and disease

The units of analysis are populations or groups of people

rather than individuals


7/27Pope et al., 1995


8/27

Advantages

Suitable exposure and outcome data may not be

available as the data is usually pre-existing

Data can be used from populations with widely differing

characteristics

Ecological studies are simple to conduct, often using

pre-existing data collected for other purposes

Disadvantages

Usually not possible to adjust for potential confounders

Links between exposure and disease seen at the aggregate

(population) level may be spurious and not seen at the

individual level (ecological fallacy)


9/27

Objectives

To examine health problem or disease

frequency

To examine association between the exposure

and health problem or disease frequency

Unit of analysis is individual

Exposure and disease status of individual is

assessed at the same time


10/27


11/27

Cross-sectional Studies measure prevalencePrevalence is the number of cases in a

defined population at a specified point in

time, expressed as a proportion of the totalpopulation at risk for the condition. The

prevalence rate for a disease is calculated as

follows:P = No. of people with disease at specified time

No. of people in population at risk at specified time


12/27

0

10

20

30

40

50

60

%

50-54 55-59 60-64 65-69

Male Female

Cork and Kerry Diabetes & Heart Disease Study, 1998


13/27

AdvantagesA cross sectional study is short term,

easy and economical to conduct

A cross sectional study generally starts

with a reference population and is

generalisable

Causal inference can be made from cross

sectional data, provided it is known that the

exposure preceded the effect or disease


14/27

DisadvantagesIt is not possible to determine in some

cases whether the exposure preceded the

condition or disease

The problem of selective survival may

be an issue

Not suitable for investigation of rare diseases

Generally requires large number of subjects


15/27

Type of

Study

Alternative

Name

Unit of

Study

Ecological

Cross-sectional

Case-control

Cohort

Correlational

Prevalence

Case-reference

Follow-up

Populations

Individuals

Individuals

Individuals


16/27


17/27

Doll and Hills Data

Lung cancer patients Controls Total

Smokers 647 622 1269

Non-smokers 2 27 29

Total 649 649 1298


18/27


19/27

AdvantagesWell suited to the study of rare disease

Relatively quick and inexpensive to conduct

Requires comparatively few subjects

Existing records can be used in

some case-control studiesNo loss to follow-up.

Allows study of multiple potential

causes of disease


20/27

Disadvantages

Relies on record or recall for

information on past exposure

(potential for recall bias)

Unsuitable for the study

of rare exposure

Selection of an

appropriate control

group may be difficult

May not be able to

establish sequence of

events


21/27


22/27

Incidence rate

Cumulative Incidence

Relative Risk

Incidence rate (IR)is the number of new cases arising in a


23/27

No. people who get disease (given time period)Sum of time each person remained under

observation and at risk of becoming a case

Usually expresses as number of cases per 1000 or per,

100,000 person years of follow-up.

Incidence rate (IR)is the number of new cases arising in a

given period in a specified population. Incidence can be

measured as follows:

Cumulative Incidence measures the denominator only at the

beginning of the study. Cumulative incidence can be measured as

follows:

IR =

CI= No. people who get disease (given time period)No. disease free people in population at risk at

the beginning of the period

Usually expresses as % risk during a defined period of

follow-up, e.g. over 1 year or 5 years


24/27

Ratio Measure

Incidence of disease in exposedIncidence of disease in non-exposed

Note: in case-control studies relativerisk is derived indirectly from the

odds ratio.


25/27

Smoking category No. stroke cases Person-yearsobservation (>8)

Strokeincidence rate

(per 100,000person-years)

Never smokes 70 395,594 17.7

Ex-smoker 65 232,712 27.9

Smoker 139 280,141 49.6

Total 274 908,447 30.2

Cigarette smoking and incidence rate of

stroke in a cohort of 118,539 women

Colditz et al., 1988


26/27

Advantages

Suitable for the study of rare exposureCan assess multiple outcomes (effects)

of single exposure

Can demonstrate temporal relationship

between exposure and diseaseAllows direct measurement of incidence

of disease in the exposed and non-exposed

population

Changes in exposure over time can be

studied

Recall and selection bias are unlikely


27/27

DisadvantagesNot suitable for the study of a rare disease,

unless a large sample size is obtained

If prospective, can be expensive and

time consuming

If retrospective, requires the availabilityof existing record

Validity of the result can be affected

by loss of follow up

cross sectinal and methods

Documents