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LMAP (Y2) > Designing Epidemiological Studies > Flashcards

Designing Epidemiological Studies Flashcards

1
Q

Outline descriptive epidemiology

A

Describes the problem often at an aggregated level
Can be used to inform later analytical research

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2
Q

Outline analytic epidemiology

A

Deploys and tests hypotheses, often at a person-level through which association can be measured and causation inferred.

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3
Q

Give examples of descriptive epidemiological investigations

A

Case report
Case series
Cross-sectional
Longitudinal
Ecological

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4
Q

What are the three epidemiological dimensions (with examples of each)?

A

Person - demography such as age, gender, occupation and disease status
Place - a hospital, geographical area, a certain community
Time - point in time, over a specified period

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5
Q

Give examples of ‘exposures’

A

Age
Gender
Occupation
Living in a particular area

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6
Q

What are outcomes usually focused on?

A

Morbidity and mortality

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7
Q

Can outcomes also be exposures?

A

Yes, e.g. hypertension can be an outcome but also an exposure/ risk factor for stroke.

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8
Q

What is a statistic?

A

A fixed value derived from a sample that estimates the value in the population (I.e. estimates a parameter)

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9
Q

What is a parameter?

A

A fixed, often unknown value, which describes an entire population

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10
Q

What is the benefit of a wider confidence interval range for populations with lower numbers?

A

It takes into account volatility in low numbers

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11
Q

What are case reports used for?

A
  • Used to communicate new diseases, presentation or findings
  • Unusual findings - used for Continuous Professional Development
  • In new diseases - multiple case reports used to form a case series
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12
Q

What is a cross-sectional study used to do?

A

Describes prevalence of a condition across a population at a single point in time
E.g. a survey
Prevalence measured may be an outcome, exposure or both
Lacks follow up - risk/temporal relationships cannot be easily determined

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13
Q

What is a longitudinal study?

A

Describes prevalence or incidence of an exposure or outcome over time
- May be made up of more than one cross-sectional analysis (aggregated data)
- May alternatively look to follow the same participants over time (person-level data)

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14
Q

What are the characteristics of ecological studies?

A

Compare groups rather than individuals (analyse only aggregate level data)
May be cross-sectional or longitudinal
Descriptive or analytical

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15
Q

What is ecological fallacy?

A

Assuming that associations between groups holds for individuals
Also known as aggregation bias

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16
Q

What is one purpose of carrying out an ecological study?

A

Usually the first step in exploring a research question - can help generate research a hypothesis

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17
Q

Ecological studies rely on primary data. True or false?

A

False, they rely on secondary data

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18
Q

What are some limitations of ecological studies?

A
  • Subject to ecological fallacy
  • Rely on secondary data collected fir different purposes that may not be directly comparable between countries or time periods
  • Not clear whether exposure preceded outcome
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19
Q

What are routinely collected data and give some examples?

A

Large administrative datasets that allow us to understand populations and their health
Form the mainstay of day-to-day demography and epidemiology in the field
Examples:
Census - every ten years so usually out of date
Local GP register
Electoral register
Hospital Episode Statistics (HES) - emergency department attendances, hospital admissions and outpatient attendances from the last ten years
Local pharmacy billing data

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20
Q

Give an example of non-routinely collected data and state some limitations of such data

A

Survey data
Expensive and time consuming

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21
Q

What is data linkage?

A

Data linkage involves joining two or more datasets together and in doing so, finding out more than was possible by analysis of either original dataset alone

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22
Q

Aside from surgery’s, what other types of samples can be used for cross-sectional studies?

A

Blood samples
Diagnostic tests
Physical measurements
*Note: As long as participants are only assessed once

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23
Q

What does a case control study involve?

A

Involves comparing individuals with a particular condition/disease (cases) with a group of individuals with the same general characteristics without the condition/disease (controls).

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24
Q

What information is obtained for both cases and controls in a case control study?

A

Information on past exposure to possible risk factors and the frequency and intensity of that exposure - this information is then compared

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25
Q

What is the ‘case definition’?

A

The eligibility criteria defined for the study

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26
Q

Give examples of sources for cases (case control study)

A

Hospitals
Clinics
Community

27
Q

What must be considered when sourcing controls and assessing exposures for the case control study?

A

Controls should come from the same study population as the cases
Should be representative of the population at risk

Exposures should be measurable with similar accuracy to exposure in the cases

28
Q

What is recall bias?

A

People with a condition may be more likely to accurately report exposures (as they want to find the root cause of their condition) than people who don’t have a condition

29
Q

What are the advantages of a case control study?

A

Good for studying rare diseases
Relatively inexpensive to conduct
Quick to obtain data (can assess exposure and outcome at the same time)

30
Q

What are the disadvantages of a case control study?

A

There may be bias associated with exposure assessment
Difficulty selecting control group
Limited to assessing just one outcome
Can’t provide information about temporal relationship between exposure and outcome

31
Q

Why do studies often allocate two or more controls per case in a case control study?

A

Number of cases being studied is often limited by rarity of the disease being studied
Statistical confidence can be increased by allocating more than one control per case

32
Q

How do you calculate the odds ratio in a case control study?

A

Odds ratio = odds of exposure among cases/odds of exposure among controls

(Useful to draw a two by two table (outcome at the top, exposure at the side) and find the cross product - label A-D - AxD/BxC)

33
Q

With regards to observational studies, what type is deemed the most robust?

A

Cohort studies

34
Q

What do cohort studies typically involve?

A

A group of individuals without a disease who are observed over a period of time to see what happens to them

35
Q

What is a cohort study also known as?

A

Longitudinal study

36
Q

What is the first step in conducting a cohort study?

A

Selecting a target population and assessing their exposure status

37
Q

What is the defining characteristic of cohort studies?

A

They track people forward in time from from exposure to outcome
You always assess exposure prior to disease

38
Q

What is important to consider when selecting the target population for a cohort study?

A

If you want to study a chronic disease associated with ageing, you may want to restrict your target population
The exposure of interest may be rare so you may need to target a specific population (e.g. farmers and pesticide)
You should initially attempt to identify as many subjects as possible before adding restrictions - want findings to be generalisable

39
Q

Give some ways the cohort for a cohort study can be assembled

A

Geographic region
Occupation
Based on disease
Risk group
Birth cohort - e.g. year of birth

40
Q

Why are multiple exposures assessed at the start of a cohort study?

A

Because when you get to the analysis stage, you want to be able to control for other exposures

41
Q

Give some characteristics of exposures in cohort studies I

A

Must be well-defined
Can be binary or all individuals could have been exposed to different levels

42
Q

What methods are used to assess exposure in cohort studies?

A

Self-report
Physical measurement
Existing records, e.g. medical records, census data

43
Q

What sources can outcome measures be obtained from in cohort studies?

A

Routine surveillance, e.g. cancer registry data
Death certificates
Medical records
Directly from participant

Note: method to obtain outcome must be the same for both the exposed and unexposed groups

44
Q

What is a historical cohort study?

A

A study where the exposures and outcomes of a group of individuals are measured contemporaneously or extracted from health records.

45
Q

Why are historical cohort studies typically lower quality than prospective cohort studies?

A

There is a greater risk of both selection and information biases

46
Q

What are randomised control trials?

A

Randomised control trials are experimental studies which compare and assess the effectiveness of two or more treatments to see if one treatment is better than another

47
Q

What is a key requirement for randomised control trials?

A

There must always be a comparator group which acts as the control

48
Q

What does ‘intention to treat’ mean in randomised control trials?

A

The process of statistically analysing patients’ outcomes using the original groups to which they were allocated, irrespective of whether they took the medicine or not

49
Q

Define ‘blinding’

A

A procedure where one or more parties in a trial are kept unaware of which treatment participants have been assigned to

50
Q

Why is blinding performed in a trial?

A

To avoid and prevent conscious or unconscious bias in the design and delivery of a clinical trial

51
Q

What are the sources of bias in a clinical trial?

A

Patient being treated
Clinical staff administering the treatment
Physician assessing the treatment
Team interpreting results

52
Q

What type of bias are double blind trials particularly useful at preventing?

A

Performance and detection bias

53
Q

What is performance bias?

A

Refers to systematic differences between groups in care that is provided, or in exposure to factors other than the exposure of interest

54
Q

What is detection bias?

A

Refers to systematic differences between groups and how outcomes are determined

55
Q

True or false: Blinding increases the chance of study withdrawal

A

False - blinding helps prevent withdrawal from studies

56
Q

What is ‘power’ and how is it affected by sample size?

A

The power of a study is its ability to detect and effect or an association
Power increases with increasing sample size

57
Q

What are the three main statistical factors that influence sample size?

A
  1. Difference between the groups you’d be looking to investigate - a larger difference will require a smaller sample size. E.g. a 5mmHg or 10mmHg difference in BP
  2. The study’s power - aim for 80% - higher the power, larger the sample size
  3. Study’s alpha - sample size - decreasing alpha will increase your sample size
58
Q

What else must you consider when calculating sample size?

A

Loss to follow-up
Not unusual to lose 15% of participants
Longer the study - usually higher the loss

59
Q

What is Type I error?

A

False positive

60
Q

What is the p-value?

A

The probability of getting your results if actually there’s no real difference - probability of a false positive

61
Q

What is a Type II error?

A

False negative

62
Q

How do we combat type II errors?

A

Increasing power - however this comes with a cost (increased sample size - increased cost and complexity)

63
Q

Why may p-values not be as useful as people think?

A
  • If you run multiple analyses, for example 20 statistical tests in your study, it’s likely that one of your analyses will come back around the p = 0.05 mark.
  • The p-value is often mistaken as implying clinical significance. It doesn’t. All a p-value tells you is how likely your findings are due to chance alone.
64
Q

What should you consider when interpreting p-values?

A
  • Where you have p-values 0.01 - 0.10, interpret them with caution. Where you have consistent findings over multiple tests <0.05 or single tests <0.01 then it gives some reassurance that chance alone is less likely to be playing a part
  • Always consider clinical significance separately

LMAP (Y2) (8 decks)

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