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TROP936 > Epidemiology study designs and measures > Flashcards

Epidemiology study designs and measures Flashcards

1
Q

What are the defining features of a cross sectional study?

A

A survey of a random sample of people (aimed to be representative of the population) at a given point of time. This is usually to find associations and risk factors for disease by a questionnaire on behaviour. Cannot be used to find causation.

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2
Q

What is an example of a cross sectional study?

A

How many people in area X have heart disease? Taking a sample of the population can find prevalence of disease at that time and use questionnaires to find associations (Do you smoke?/How much red meat do you eat per day?/Do you exercise?/What are your feelings towards your local health service?)

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3
Q

What are the advantages of a cross sectional study?

A
  • Many variables compared at ones
  • Quick and cheap
  • Useful when routine data isn’t available
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4
Q

What are the disadvantages of a cross sectional study?

A
  • Not useful for a disease with a short duration (many will be missed in a snapshot study/will get different results every time you check)
  • Cannot find causality
  • Easy to introduce sampling and collection bias
  • Can’t find incidence
  • Questionnaires can often require a pilot study
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5
Q

What are the defining features of a cohort study?

A

Used to find the effect of an exposure. Take 2 groups of people with a disease and varying degrees of exposure to something you suspect is affecting the disease and follow over time. Longitudinal study where comparisons are taken continuously. Association between risk and outcome usually measured as risk ratio/odds ratio.

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6
Q

Name a type of descriptive observational epidemiological studies

A

Surveys

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7
Q

Name 2 types of analytical observational epidemiological studies

A

Case-control, Cohort

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8
Q

Name 3 types of experimental epidemiological studies

A

Experiments, randomised control trials, intervention studies

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9
Q

What are cohort studies used for?

A

To test if an exposure variable is causally related to health

To measure incidence and incidence rate over a period of time

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10
Q

What are the 2 types of cohort study?

A

Prospective (2 groups, 1 exposed, follow over time to see effect)

Retrospective (2 groups, 1 has been exposed, look back in records to see effect)

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11
Q

What are the advantages of a cohort study?

A
  • Allows outcomes to be explored over time
  • Possible to study a harmful risk factor/exposure (it would be unethical to expose a group to this risk factor in an experimental setting)
  • Useful for rare exposures (because you choose based on exposure)
  • Can look at multiple exposures
  • Can measure incidence and prevalence
  • Samples aren’t biased by knowing the outcome status - samples are defined before the disease is onset
  • High validity, true incidence known
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12
Q

What are the disadvantages of a cohort study?

A
  • Can be very time consuming and expensive
  • Behaviours can change, control groups can become exposed or exposed group stop being exposed
  • Change in measurements/classifications etc can occur over the time the study takes (can cause misclassification
  • Selection bias, consent bias
  • Loss to follow up, people might bail (might introduce bias, by which people bail)
  • Not suitable for rare diseases, might never come up
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13
Q

What is the Framingham Heart Study and why was Framingham a good place for it?

A

Longest running cohort study of cardiovascular disease in USA.

Framingham:

  • Adequate size
  • Stable employment and population
  • Local authority kept a list of residents
  • One health facility so easy to follow up
  • Variety of socioeconomic and ethnic subgroups
  • Previous successful study of TB
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14
Q

What analyses are associated with cohort studies?

A
  • Incidence
  • Incidence rates
  • Risk ratios
  • Odds ratios
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15
Q

Define incidence

A

Number of instances of illness commencing during a specified period

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16
Q

What is incidence rate?

A

Number of people who develop a disease in 1 year/Average number of people in one population

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17
Q

What is cumulative incidence?

A

Number of people who develop disease in period/Number of people at risk of getting the disease at start of period

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18
Q

You want to find out if there are patterns, relationships & associations. Which study?

A

Comparative

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19
Q

You want to know how widespread the event occurrence (prevalence) is. Which study?

A

Cross-sectional

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20
Q

You want to know how quickly an event is occurring (incidence). Which study?

A

Cohort study

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21
Q

Which factors are causing the event?

A

Case control study

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22
Q

How effective is an intervention at controlling/preventing/managing an event?

A

RCT/Intervention study

23
Q

What’s the difference between a case-control study and a retrospective cohort study?

A

In a case-control study the investigator splits the group by disease and examines how exposures differ.

In a retrospective cohort study the investigator splits the group by exposure and examines how outcomes differ.

24
Q

What are the usual sources of controls for a case-control study and what are their limitations?

A

Hospital - ill by definition, increased number of smokers and alcohol abusers

Relatives - very (too) willing to participate, genetic similarity, anxiety related to the case individual

General population - unwilling to participate, uncooperative

25
Q

What is the structure of a case-control study?

A

Group with outcome + group without outcome and you look back in time to see who is exposed to the variable of interest. Used to test if an exposure variable is related to health problem.

26
Q

Which analyses can you do from case-control data and which can you NOT do?

A

Can test the null hypotheses (chi-sq and fishers exact test) and measure the magnitude of the difference (odds and odds test)

Can NOT measure risk because we chose case and control so there’s no “at risk population”

27
Q

What do you have to think of when choosing controls for a case-control study?

A

Without the disease but would have been chosen for the cases if they had had the disease (e.g. totally comparable to cases)

28
Q

What do you need to think of when defining cases for a case control study?

A

Look for previous definitions in literature

Needs to be reproducible by anyone so needs to be a clear definition

Precision needs may vary (epidemic vs clinical trial)

Clearly list the criteria needed for a positive diagnosis

Include certainty of diagnosis

Include severity of disease

29
Q

What is the difference between a risk and a hazard?

A

A hazard is something that if you are exposed to it, increases your chance of a negative outcome (e.g. being bitten by a plasmodium infected mosquito).

A risk is the probability that an outcome will occur within a given time period (e.g. the risk of developing malaria is lower for children that sleep under a mosquito net).

30
Q

What is the difference between risk ratio and rate ratio?

A

Risk ratio is the ratio of prevalence of disease (risk for main interest group / risk for comparison group).

Rate ratio is the ratio of incidence of disease (incidence rate in interest group / incidence rate for comparison group).

31
Q

If the variables depend on each other, where does each variable go in a contingency table?

A

Exposure variables are represented as rows, outcome variables as columns.

32
Q

What does it mean if the risk difference (attributable risk) is negative?

A

The exposure is protective.

33
Q

What is the advantage of presenting data as relative risk over odds ratio?

A

More intuitive and easily understood.

34
Q

What are the advantages of a case-control study?

A
  • Can be conducted relatively quickly
  • Good for rare conditions
  • Good for conditions with a long latency period
  • Good where the population isn’t stable and follow up is difficult
  • Good where multiple exposures
35
Q

What are the disadvantages of a case-control study?

A
  • Can introduce bias when choosing controls (selection bias)
  • Information collected from the case and control aren’t comparable because of interviewer bias (knows status so might capture/interpret information incorrectly) and recall bias (cases usually have a better record of information relating to the study and have a better knowledge of the disease)
36
Q

Someone calculated the odds ratio after people got ill at their dinner party and they suspected the cheesecake. The odds ratio was 9. How would you interpret this result?

A

Of the people that got ill, they were 9 times more likely to have eaten the cheesecake than those that didn’t get ill.

37
Q

Between which type of variables is the chi-squared test used to test for significance difference?

A

Catagorical

38
Q

How do you calculate risk?

A

cases/at risk population

39
Q

How do you calculate risk ratio?

A

Risk for main interest group / risk for comparison group

40
Q

How do you calculate rate ratio?

A

Rate for main interest group / rate for comparison group

41
Q

Why can’t you calculate risk for a case-control study?

A

You don’t know the size of the at risk population, case and control groups were chosen by investigator.

42
Q

If the odds of exposure to cheesecake for those that got ill was 3:1, what does that mean?

A

For every 4 people that got ill, odds are that 3 of them ate the cheesecake.

43
Q

How do you calculate odds ratio?

A

Odds of exposure to risk if got disease / odds of exposure to risk if didn’t get disease

44
Q

How do you calculate chi-squared?

A

Sum of (observed values-expected values)^2 / expected values

45
Q

How do you calculate the expected values for a contingency table?

A

(row total x column total) / population total

46
Q

What is an appropriate research question for a comparative cross-sectional survey?

A

What is the magnitude of the problem?
What is the prevalence?
What are the risk factors?
Are the risk factors associated with the outcome?

47
Q

What is an appropriate research question for a cohort study?

A

What is the relative risk?
What is the prevalence?
What is the inferred cause of the disease?
What is the association between risk factor and outcome?

48
Q

What is an appropriate research question for a case-control study?

A

What is the strength of the association between a risk factor and an outcome?
Does exposure to a specific risk factor differ between the cases and controls?

49
Q

What is an appropriate research question for an RCT?

A

What is the effect of a therapy?

What are the causes?

50
Q

What is the difference between incidence and prevalence?

A

Prevalence is the proportion of the population that have the disease at a given time whereas incidence is how many new cases in a given amount of time. So, prevalence tells you how widespread the disease is and incidence tells you the likelihood of someone new contracting it.

51
Q

When is a cross-section survey appropriate and when isn’t it?

A

Good when routine data isn’t available, you want to find prevalence or associations with disease.
Not appropriate if you want to find causation.

52
Q

When is a cohort study appropriate and when isn’t it?

A

Good for finding causal links between a harmful exposure and an outcome that would be unethical in other situations, or for rare exposures (because you choose the exposure group).
Inappropriate for rare diseases or those with long latent periods. Also for an unstable population.

53
Q

When is a case-control study appropriate and when isn’t it?

A

Appropriate for finding if an exposure is related to an outcome, especially appropriate if the disease is rare (because you choose by cases).
Inappropriate if you want to know risk or incidence.

54
Q

When is a RCT appropriate and when isn’t it?

A

Appropriate for finding causal links between a therapy and an outcome, or a risk and an outcome.
Inappropriate for prevalence/incidence data, e.g. finding out about the wider population.

TROP936 (4 decks)

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