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Cancer > Study design > Flashcards

Study design Flashcards

1
Q

What factors must be considered when designing a research study?

A
  • Research question
  • Study design
  • Subjects
  • Data
  • Analysis
  • Interpretation
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2
Q

What are two quantitative study designs?

A
  1. Experimental
    - Controlled trials
    - Quasi experimental
  2. Observational
    - Cross sectional
    - Cohort
    - Case-control
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3
Q

What are types of observational studies?

A
  1. Descriptive: used to formulate a certain hypothesis:
    - examples: case-studies cross-sectional studies, ecological studies.
    example: what is the prevalence and trends in obesity
  2. Analytical: used to test hypotheses: • examples: case-control, cohort.

Example: How much exercise is necessary to reduce risk of specific diseases?

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4
Q

What are the various study populations?

A

Target Population: Group of individuals about whom you want to make inferences

Source Population: Group of individuals from whom the study population is drawn

Study Population: Group of individuals that serve as study participants

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5
Q

What is a confounding variable?

A

A distortion of the measure of association between the exposure and the outcome due to the mixing of the effect of the exposure with another risk factor.

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6
Q

What determines what type of study is pursued?

A

The research question

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7
Q

How can a valid link be made from the target to the study population?

A

First answer what is the study question? and
who do the study results apply to? Think of target population

Choose a source population that reflects
the target population

Choose study participants who represent the source population

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8
Q

What does a systematic error lead to?

A

more chance of finding the wrong result

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9
Q

What does a random error lead to?

A

less chance of finding the true result

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10
Q

What does choosing between observational methods depend upon?

A

Depends on:
• how rare the outcome is
• what data exists for the population of interest
• whether the temporal relationship is important
i.e. Exposure leads to Outcome (cause to effect)
• how quickly you want the answer
• money / resources

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11
Q

What are ecological studies?

A

Studies that investigate risk factors of health outcomes in which the unit of analysis is at the group level rather than the individual.

Group measures (exposure and or outcome) can include:
• summary measures of a group (mean, average rate)

• environmental factors (air pollution, hours of sun-light, fast-food shops)

i.e. something that is not measured at the individual level

Examples: • Time trends, geographic comparisons

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12
Q

What are the advantages and disadvantages of ecological studies?

A

Advantages:
• Easy to do
• No individual data necessary
• Good to generate ideas about potential associations

Disadvantages:
• No information on the individual level
• Not able to account for other factors that might explain the association e.g ecological fallacy

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13
Q

What is ecological fallacy?

A

The ecological fallacy occurs where an analysis of group data is used to draw conclusions about the individual.

Example:
The average salary is higher in countries that sell more reading glasses

Therefore if you wear reading glasses you are likely to have a higher salary

Likely to be due to other factors that are not taken into account (confounders)

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14
Q

What process is involved in a cross sectional study?

What measure of association can be used?

A

The process

  • Select a sample (representing the population of interest)
  • Measure exposure and outcome variables at the same time
  • Determine prevalence

Measure of association: odds ratios

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15
Q

What are the strengths and weaknesses of a cross sectional study?

A

Strength:

  • fast and inexpensive
  • immediate answers – no follow-up time
  • no loss to follow-up (but can have non-responders)

Weaknesses:
• can’t determine temporal relationship
• not good for rare exposures or outcomes
• bias can be a problem - measurement bias, survivor bias

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16
Q

What process is involved in a cohort study design?

A

The Process

  • Start with the POPULATION of interest
  • Identify or assemble a cohort
  • Measure risk factor(s) and potential confounders
  • Measure the outcome over the follow-up period
17
Q

A cohort study can be either…

A

Prospective:

Start with assembling a cohort, measure risk factors then follow over time to measure outcomes

Retrospective (historical):

Identify a suitable cohort (from the past), collect risk factor data measured in the past, collect subsequent outcome data

18
Q

How can risk be determined using cohort studies?

A

Absolute risk

Relative risk (RR) =
Incidence (exposed) Incidence (not exposed)
19
Q

What are the general strengths of cohort studies?

A 
Cohort studies (in general):
• Can establish sequence of events
  • Can assess risk of multiple outcomes at the same time
  • Can estimate incidence (how many new events within a certain time)
  • Able to directly calculate absolute and relative risk
20
Q

What are the strengths of prospective cohort studies?

A
  • Can control who is in the cohort

* Lower risk of bias (exposures measured before outcomes)

21
Q

What are the strengths of Retrospective cohort studies?

A

• More efficient: less time, less costly

22
Q

What are the general weakness of Cohort studies?

A
  • not a controlled experiment - so can’t claim ‘causation’
  • difficult to control for all other confounding factors
  • expensive – often require large sample
  • not good choice for rare outcomes
23
Q

What are the weaknesses of prospective studies?

A
  • not timely, long follow-up

* potential loss to follow-up (can lead to bias)

24
Q

What are the weaknesses of Retrospective studies?

A
  • knowing the outcome might lead to bias
  • limited to data already collected
  • little control over who is in the cohort
25
Q

What process is involved in a case control study?

A
  • Select a sample of ‘cases’ (i.e. people who have the condition/disease)
  • Select a sample of ‘controls’ (i.e. people without the disease but who
    have the same chance of having the disease)
  • Measure (past) exposure to risk factors of interest
26
Q

In case control studies, an issue with selecting study subjects is that results may be biased if exposures are different due to the selection process. How can this be avoided?

A

CASES – all those who develop a disease (or random sample):

  • Cases from a hospital
  • Cases on a population registry (i.e. cancer registry)
  • Best to be new cases

CONTROLS must come from a source population with similar chance of being exposed to the risk factor of interest:

  • Hospital-based controls with different disease
  • Population controls (good if cases are from a registry)
27
Q

How many controls are needed in case-control studies?

A
  • Statistical power of higher for case-control studies so sample sizes are lower
  • 1:1 selection is often OK especially when number of cases is large
  • Ratio of controls can be increased when number with outcomes is low or when the proportion likely to be exposed is low
  • 1:2 is common
  • not much power gained by increasing >3
28
Q

How should matching be pursued in case-control studies?

A

Controls can be matched with cases to ensure they are comparable with respect to other influencing factors (confounders)

  • age and sex matching is common - strongly linked to many diseases and exposures
  • sometimes geographical area
  • individual matching
  • frequency matching (overall proportions are the same)

Matching is not necessary but can be useful to control for strong confounders

Overmatching can ‘hide’ true effects, and can limit the analysis that can be done

29
Q

How can measurement bias occur in case-control studies?

What are possible solutions for this?

A

Information about exposures collected after the outcome is known can introduce bias:

  • different level of recall/reporting of risk factors between cases and controls
  • more details about risk factors in cases (because of the disease)
  • researchers look harder for evidence of exposure in cases

Possible solutions
• use data collected before outcome was known
• researcher blinded to outcome
• subjects asked about multiple possible risk factors
• pick controls have a disease that is linked to similar risk factors

30
Q

What are the strengths and weaknesses of case-control studies?

A

Strengths
• Useful for rare outcomes (eg specific cancers)
• Efficient/less costly than cohort study: smaller sample size; no follow-up required

Weaknesses
• Biases if cases and controls are from different populations

  • Biases due to measuring exposure after the outcome
  • Confounding due to other influential factors (not measured)
  • Can only study one outcome
31
Q

How is risk calculated in a cohort study?

A

Relative risk (RR) = Incidence in exposed/
Incidence in non-exposed
a/a+b c/(c+d)

32
Q

How is risk calculated in a CASE-CONTROL (& cross-sectional)?

A 
Odds Ratio (OR) =
Odds of exposure in cases/ Odds of exposure in controls
33
Q

What are potential errors and biases in observational studies?

A

Selection bias
• Study population differs from broader population in terms of the relationship between exposure and outcome

Measurement (information) bias
• Measurement of exposure or outcome differs between groups

Confounding:
• Other factors associated with both exposure and outcome can distort the main effect if not taken into account

Random error:
• Due to natural variation in the population / less precise measurements

Cancer (20 decks)

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