Descriptive Analytical

Case report

Case series

Cross section

Ecological

Case control

Cohort

Observational Experimental

Study Designs

Animal Experiment

HumanInterventionClinical trial

Observational Studies

• non-experimental

• observational because there is no individual intervention

• treatment and exposures occur in a “non-controlled” environment

• individuals can be observed prospectively, retrospectively, or currently

Case Reports

• Careful and detailed report by one

or more clinicians of the profile of

a single patient• e.g. previous un-described disease

• e.g. unexpected link between diseases

• e.g. unexpected new therapeutic effect

• e.g. adverse events

Case Reports

• exposures (i.e. a case report gave the clue OC use increases the risk of venous thromboembolism.

• “Luck” in being the first to encounter an interesting case. • Rigor in diagnosis, testing and documentation of clinical findings

Strengths• over one million case reports indexed on

Medline.• uses language that is familiar to clinicians and

easy to interpret.• useful reminder about conditions, diagnoses

etc. that are rarely seen in most practices.• for researchers, case reports generate

hypotheses that can be tested using other study designs.

Case Reports

Limitations:

• No appropriate comparison group.• Cannot be used to test for presence

of a valid statistical association.• Since based on the experience of

one person:--- presence of any risk factor

may be purely coincidental--- not a true epidemiologic

design

• Tendency to publish "gee whiz" reports of strange conditions that have little relevance to daily practice.• Some authors erroneously try to imply causation, therapeutic benefits, etc.

Case report from Medline

Authors: Smart ER. Macleod RI. Lawrence CM.Title: Allergic reactions to rubber gloves in dental patients: report of three cases.

Source: British Dental Journal. 172(12):445-7, 1992 Jun 20.

Abstract:Three cases of allergy to rubber are described, in which the patientsexhibited peri-oral rashes following dental treatment by personnelwearing latex rubber gloves. Two of the patients were aware of possible allergy to domestic rubber products but did not reveal this as part of their medical history. With the increase in numbers of dentists wearing rubber gloves it is probable that there will be many more such cases reported in the future. Rubber products must then be added to the list of potential allergens which may be of some import to the practice of dentistry.

Case Series• Experience of a group of patients with a

similar diagnosis.• 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

• Advantages

• Cannot study cause and effect relationships.

• Cannot assess disease frequency.

Case SeriesStrengths:

• Useful for hypothesis generation.

• Used as an early means to identify the beginning or presence of an epidemic.

• Can suggest the emergence of a new disease (i.e. AIDS).

• Informative for very rare disease with few established risk factors.

Case Series

Limitations:

• Lack of an appropriate comparison group

• Cannot be used to test for presence of a valid statistical association

• Not a true epidemiologic design.

Cross-sectional studies

• An “observational” design that surveys exposures and disease status at a single point in time (a cross-section of the population)

time

Study only exists at this point in time

Cross-sectional Design

time

Study only exists at this point in time

Studypopulation

No Disease

Disease

factor present

factor absent

factor present

factor absent

Prevalence

Cross-sectional Studies

• Often used to study conditions that are relatively frequent with long duration of expression (nonfatal, chronic conditions).

• It measures prevalence, not incidence of disease.• Not suitable for studying rare or highly fatal

diseases or a disease with short duration of expression.

Cross-Sectional Studies

Strengths:• Provides prevalence estimates of

exposure and disease for a well-defined population.

• Easier to perform than studies that require follow-up (hence relatively inexpensive).

• Can evaluate multiple risk (and protective) factors and health outcomes at the same point in time.

Strengths:• May identify groups of persons at

high or low risk of disease

• Can be used to generate hypotheses about associations between predictive factors and disease outcomes

Cross-Sectional Studies

Limitations:• Prevalent rather than incident (new)

cases are used – the exposure could be associated with survival after disease occurrence, rather than development of the disease

• Temporal sequence between exposure and disease cannot be established

* i.e. Which came first, chicken or the egg?

Cross-sectional studies

• Usually don’t know when disease occurred.

• Rare events a problem. Quickly emerging diseases a problem

Health Survey

• Survey (n): Information gathered by asking a group of individuals the same questions related to their characteristics, attributes.

 

• Survey (v): The process of collecting such information

Knowing what to expect

• A good survey has the potential to reach a large number of respondents;

• Generate standardized, quantifiable, empirical data - as well as some qualitative data; and offers confidentiality / anonymity

• Credible data, however, can be difficult to generate

Surveys can be:

– Descriptive:

These surveys pretty much do what they say - they describe. The goal is to get a snapshot - of your ‘respondents’

 – Explanatory:

These surveys go beyond description and attempt to establish why things might be the way they are

• Surveys can also involve populations or samples of populations:

– Census: This is a survey that does not rely on a sample. A census surveys every single person in a defined or target population

 – Cross-sectional surveys: This type of

survey uses a sample or cross-section of respondents selected to represent a target population

• Surveys administration:

– Face to face– Telephone– Self-administered.– Electronic e.g. E mail

• Conducting a survey capable of generating credible data requires: – thorough planning– meticulous instrument construction– comprehensive piloting– reflexive redevelopment– deliberate execution– and appropriate analysis

• Survey questions can either be open or closed:

– Open questions: Open questions can generate rich and candid data, but it can be data that is difficult to code and analyze.

– Closed questions: These questions force respondents to choose from a range of predetermined responses, and are generally easy to code and statistically analyze.

Considerations in Survey Construction

• Providing clear background information and lucid instructions.

• Logical organization.

• Comprehensive coverage without undue length.

• User friendly and aesthetically pleasing layout and design.

Ecologic Studies• Measures that represent characteristics of

entire populations are used to describe disease and to postulate causal associations.

• Measure of interest is correlation between exposure rates and disease rates among different groups.

• Correlation coefficient (denoted as r)Range of r is from –1.0 to 1.0R evaluated in relation to difference

from 0.

Ecologic StudiesStrengths:• Cheap, quick, and simple (generally

make use of secondary data)

Limitations:• Cannot link exposure-disease

relationship at the individual level

• Uses average exposure levels rather than actual levels of exposure

• Inability to control for confounding factors

EXAMPLE: Country A: Country BPrevalence-Hypertension 30% 20%Average Salt Consumption Moderate Low

Country A Country BPerson Salt Intake Hyp. Salt Intake Hyp. 1 1 Yes 1 Yes 2 1 Yes 1 Yes 3 1 Yes 1 No 4 3 No 1 No 5 3 No 1 No 6 3 No 2 No 7 3 No 2 No 8 5 No 2 No 9 5 No 2 No 10 5 No 2 No

Avg. 3.0 30% 1.5 20%

Ecologic Studies

The “Ecologic Fallacy:”

• Erroneous conclusions based on grouped data

• Patterns observed on the aggregate level are not observed on the individual level