L19: Observational Studies

Question 1

What is an observational study?

Answer 1

investigator did not assign exposure

Question 2

What is an observational study that has a comparison group vs no comparison group?

Answer 2

Have comparison group -> analytical study (cohort study, case control study and cross sectional study)

No comparison group -> descriptive study (case report, case series and ecologic study)

Question 3

What is an ecologic study?

Answer 3

• unit is population, not individual

- uses aggregated/group level data

Question 4

What is ecologic fallacy?

Answer 4

• data collected at group level assumed to apply to individual level

Question 5

What are the strengths and limitations of an ecologic study?

Answer 5

Strengths

• inexpensive, easy to conduct (usually using secondary data eg published statistics)
• hypothesis generating

Limitations

• cannot link exposure to outcome in individuals
• confounding by other variables

Question 6

What is a case report and case series?

Answer 6

• involves individuals
• careful, detailed report of the profile of a single patient (case report) or series of patients (case series), with respect to factors that could be related to illness or outcome -> usually on an unusual disease
• often the first alert by observant HCP

Question 7

What are the strengths and limitations of case report and case series?

Answer 7

Strength
- hypothesis generating

Limitation
- no comparison group

Question 8

What is a cross-sectional study?

Answer 8

• snapshot in time
• information on presence of absence of exposure and outcome of individuals assessed simultaneously at one point in time
• provides info on prevalence

Question 9

What are the strengths and limitations of a cross-sectional study?

Answer 9

Strength:

• efficient in time and money
• many outcomes and risk factors can be assessed
• no loss to follow up

Limitations:
- unclear temporal relation between exposure and outcome -> difficult to establish causal relationship

Question 10

What is a cohort study?

Answer 10

• follow 2 or more groups from exposure to outcome

- selection based on exposure status

Question 11

What are the strengths and limitations of a cohort study?

Answer 11

Strength:

• clear temporal sequence between exposure and outcome
• can study several outcomes associated with a single exposure
• ideal for studying rare exposures
• can directly measure incidence of outcome

Limitations

• inefficient for studying rare outcomes (might not find it, need v large sample size)
• inefficient for outcomes that take v long to develop
• time consuming, costly
• potential for bias due to loss to follow up

Question 12

What are two types of cohort studies?

Answer 12

Prospective cohort study - outcome has not occurred at initiation of study

Retrospective cohort study - outcome already occurred at initiation of study (but still look at exposure first)

Question 13

What are the strengths and limitations of a prospective vs retrospective cohort study?

Answer 13

Prospective

• more control over quality and quantity of data -> less potential for bias
• more time consuming, expensive

Retrospective

• less control over quality and quantity of data -> greater potential for bias
• less time and money needed

Question 14

What is a case control study?

Answer 14

• select based on outcome status

- look back to see if exposed or not (retrospective)

Question 15

What are some sources of cases and controls?

Answer 15

Cases

• medical facilities
• disease registries

Controls

• hospitalized patients
• general population

Question 16

What are the advantages and disadvantages of using hospitalized patients as controls?

Answer 16

adv:
- convenient
- relatively inexpensive to identify and interview
- cases and controls likely similar in accuracy of recall
- generally high level of cooperation

disadv:
- disease for which they hospitalized for may be associated with risk factors under study
- selection factors leading to hospitalization may differ between cases and controls (referral patterns)

Question 17

What are the advantages and disadv of using general population as controls?

Answer 17

adv:
- ensures comparability

disadv:
- difficult to enumerate all members of population as basis for selecting individuals
- difficult to gain cooperation
- relatively expensive to identify and interview
- may not recall exposure w same degree of accuracy as cases

Question 18

What are the strengths and limitations of a case control study?

Answer 18

strength:

• ideal for studying rare outcomes
• efficient for studying outcomes that take long time to develop
• can study multiple exposures for a single outcome
• efficient in time and money (bc outcome alr occurred)

limitations

• inefficient for rare exposures
• selection of appropriate control is difficult
• data on exposure may be difficult to obtain
• cannot estimate outcome rate directly, but can use estimate measures
• more potential for bias bc outcome status is known when assessing exposure status

Question 19

What is a nested case control study?

Answer 19

case control study nested within a cohort study

Question 20

What are the strengths and limitations of a case control study?

Answer 20

strengths;

• efficient in time and money (useful for costly measurements archived at beginning and preserved for later analysis for only cases and controls, not the whole cohort)
• preserves all adv of cohort studies that result from collecting exposure data first before outcome
• avoids potential bias of conventional case control studies that draw cases and controls from different populations

limitations:
- many research qns not amenable to strategy of storing materials for later analysis on a sample of the study subjects
- when data available for entire cohort at no additional cost, nothing is gained by studying only a sample of controls -> whole cohort should be used

Question 21

What do we refer to when reporting observational studies?

Answer 21

STROBE statement