Observational Studies Flashcards

1
Q

What are the two type of study designs?

A
  1. Observational

2. Interventional

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2
Q

What are observational studies?

A

We just observe and participants do not change.

They are either:

  • Descriptive
  • Analytical
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3
Q

Give an example of a descriptive study

A

Cross sectional studies (surveys)

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4
Q

What is the aim of descriptive studies?

A
  • Describe prevalence of disease (which is the number of cases in a population at a given time)
  • How disease varies over time, place by place or characteristics of individual
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5
Q

Advantage of descriptive studies

A
  • Able to identify patterns and trends to allow for further research/hypothesis - suggest clues to cause of disease
  • Allow us to see burden of disease (impact of disease - financial, mortality and community)
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6
Q

Disadvantage of descriptive studies

A
  • Does not provide sufficient evidence to infer causality as they are mostly retrospective and do not account for many details such as confounders
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7
Q

How can cross sectional studies be descriptive or analytical?

A

They can be descriptive - aiming to assess the burden of disease

They can be analytical - aim to explain observed pattern of disease by examining its relationship with possible aetiological factors

Most cross sectional studies have a mixture of both

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8
Q

Describe a cross sectional study

A
  • Use to measure prevalence of a disease (via medical records, questionnaires etc)
  • Carried out by collecting data (characteristic and disease status) at one point to see if subjects with certain exposure/characteristic have higher disease prevalence
  • Can be descriptive and analytical
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9
Q

Advantages of cross sectional studies

A
  • Suited to common diseases
  • Relative quick to carry out and is cheap
  • Can give estimates of prevalence of disease
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10
Q

Disadvantages of cross sectional studies

A
  • Unable to measure incidence (just prevalence)
  • Not suitable in rare diseases as a very large sample is needed
  • Need to ensure that you avoid selection bias
  • Exposure/characteristics may change in time or disease progression
  • Reverse causality (information on exposure preceding disease is unknown). This occurs as exposure/characteristics and disease status are measured at one point in time so its hard to tell what came first.
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11
Q

Examples of analytical studies

A
  • Longitudinal/cohort studies
  • Case-control studies
  • Cross-sectional studies
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12
Q

What is the aim of analytical studies?

A

The aim is to examine associations between the presence of diseases in individuals and populations with potential causative factors.

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13
Q

Advantages of analytical studies

A
  • Gives equal information on both controls (free of disease) and cases (those with disease)
  • Gives greater clues ti causality than descriptive studies
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14
Q

Disadvantages of analytical studies

A

Still unable to infer causality due to possible confounders/unknown variables

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15
Q

Describe a Longitudinal/Cohort Studies

A
  • Analytical study
  • Used to see if those with certain characteristic who do not have disease go on to DEVELOP the disease more frequently than those who do not have the disease
  • Subjects are divided into two groups (those who have characteristic and those who do not)
  • When measuring exposure status of subjects they must be free of the disease under investigation
  • Two types: prospective cohort study or retrospective cohort study
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16
Q

What are the two types of Longitudinal/Cohort Study?

A
  1. Prospective Cohort Study

2. Retrospective Cohort Study

17
Q

Advantages of Longitudinal/Cohort Studies

A
  • No recall bias as it doesn’t rely on subjects memory
  • Exposure is measured before causality, therefore less bias and increased reliability in causality
  • More than one disease can be measured for any one exposure
  • Can calculate incidence rates
  • Can calculate relative risk
  • Potential to give more information leading to nested case control study
18
Q

Disadvantages of Longitudinal/Cohort Study

A
  • Can take a long time and be expensive
  • Collection of data may alter behaviour - observer effect
  • Ensure definition of exposure or disease is consistent
  • Losses by follow up may introduce selection bias (patients who are healthy leave)
  • Not suitable for low incidence disease
19
Q

What is a prospective cohort study?

A

Population is studied for the presence of a fixed or modifiable exposure, the population is followed up and the incidence of disease in exposed individuals is compared with the incidence of those who are not exposed.

20
Q

What is retrospective cohort study?

A

Past medical records are used to identify a population and obtain data on previous exposures. The incidence of disease in exposed individuals is compared with the incidence in those not exposure.

21
Q

Compare retrospective and prospective cohort studies

A

Retrospective cohort studies are cheaper and less time consuming

22
Q

Describe case control studies

A
  • Subjects are divided into two groups -those who have disease and those who do not
  • This aims to identify cases (with disease) and controls (without disease) and then measure the prevalence of a particular exposure
  • This aims to see if cases had a characteristic more frequent than the control
  • If exposure is more common in cases than controls, it may be a risk factor
  • If exposure is more common in control than cases, it may be a protective factor
23
Q

What does risk factor and protective factor mean?

A

If exposure is more common in cases than controls in case control study it is a risk factor.

If exposure is more common in control than cases in a case control study it is a protective factor.

24
Q

Advantages of case control studies

A
  • Cheap and quick to do
  • Efficient for rare diseases or disease with low incidence
  • Can investigate a range of risk factors
  • Can work out the odds ratio - which is even more accurate in rare diseases and can be interchanged with relative risk
25
Q

Disadvantage of case control studies

A
  • Prone to selection and information bias, such as recall bias as the exposure is measured after the disease develops
  • Can’t calculate relative risk as there are no cases of incidence
  • Not suitable for exposures/ characteristics that are rare
  • Not suitable for diseases with several main exposures/ risk factors
  • Need to ensure data collection is not influenced by knowledge of exposure to prevent measurement bias
  • Disease may affect exposure having a reverse causal relationship.
26
Q

What is a nested case-control study?

A

This is when a case-control study is inserted into a cohort study.

A subset of controls (from cohort study without disease) are compared to incidence cases (from cohort study - those who developed the disease).

The cases of cohort study become the nested case control study.