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1st term: CPH/PPD > Study designs > Flashcards

Study designs Flashcards

1
Q

What are the 3 types of study designs?

A
  • RCT
  • Cohort study
  • Case-control study
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2
Q

What is a RCT?

A

= randomised control trial

Participants randomly allocated to intervention or control group (e.g. standard treatment or placebo)

Practical or ethical problems may limit use

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3
Q

What is a cohort study?

A
  • Observational
  • prospective.

Two (or more) cohorts selected according to their exposure to a particular agent (e.g. medicine, toxin) and followed up to see how many develop a disease or other outcome.

The usual outcome measure is the relative risk.

Examples include Framingham Heart Study

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4
Q

What is a case-control study?

A
  • Observational
  • retrospective

Patients with a particular condition (cases) are identified and matched with controls.

Data is then collected on past exposure to a possible causal agent for the condition.

The usual outcome measure is the odds ratio.

Inexpensive, produce quick results

Useful for studying rare conditions

Prone to confounding

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5
Q

What is confounding in statistics?

A

a variable which correlates with other variables within a study leading to spurious results

case-control studies are vulnerable to confounders

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6
Q

How can CONFOUNDING be controlled in the design stage of an experiment?

A

Randomisation

aims to produce an even amount of potential risk factors in both populations (cases and controls)

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7
Q

How can CONFOUNDING be controlled in the analysis stage of an experiment?

A

Stratification

Adjustment or normalisation for that confounding variable to remove its effect in the study outcome

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8
Q

What are the 3 types of association?

A

DIRECT
a true association not linked by a third (confounding) variable

SPURIOUS
an association that has arisen by chance and is not real

INDIRECT
the association is due to the presence of another factor (a confounding variable)

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9
Q

What criteria are used to establish whether an association is causal?

A

Bradford-Hill criteria

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10
Q

What are the main aspects of the BRADFORD-HILL criteria used to establish causation in a study?

A

STRENGTH
The stronger the association the more likely it is to be truly causal.

TEMPORALITY
Does the exposure precede the outcome?

SPECIFICTY
Is the suspected cause associated with a specific outcome/ disease?

COHERENCE
Does the association fit with other biological knowledge?

CONSISTENCY
Is the same association found in many studies?

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11
Q

What is bias?

A

the situation in a trial where one outcome is systematically favoured

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12
Q

What is SELECTION BIAS?

A

Error in assigning individuals to groups leading to differences which may influence the outcome

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13
Q

What are the sub-types of selection bias?

A

SAMPLING BIAS
subjects are not representative of the population

VOLUNTEER BIAS
One risk factor or study aspect may make certain individuals more or less likely to volunteer for a study. This can lead to further sampling bias

NON-RESPONDER BIAS
similar to volunteer bias, but in the opposite effect. Those who do not respond to a survey (for e.g.) may disproportionately fall into one of the categories being measure over the other

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14
Q

What is prevalence/incidence bias?

A

= Neyman bias

occurs when a study is investigating a condition that is characterised by early fatalities or silent cases.

It results from missed cases being omitted from calculations

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15
Q

What is admission bias?

A

= Berkson’s bias

cases and controls in a hospital case control study are systematically different from one another

This is because the combination of exposure to risk and occurrence of disease increases the likelihood of being admitted to the hospital

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16
Q

What is recall bias?

A

Difference in the accuracy of the recollections retrieved by study participants

possibly due to whether they have disorder or not

particularly an issue in case-control studies

17
Q

What is the Hawthorne effect?

A

Describes a group changing it’s behaviour due to the knowledge that it is being studied

18
Q

What is procedure bias?

A

Occurs when subjects in different groups receive different treatment

19
Q

What is lead-time bias?

A

Occurs when two tests for a disease are compared, the new test diagnoses the disease earlier, but there is no effect on the outcome of the disease

20
Q

What is a placebo?

A

inert substance that has no inherent pharmacological activity.

It looks, smells and tastes like the active drug with which it is compared.

A placebo need not always be pharmacological. It could be procedural, for example, sham electroconvulsive therapy (ECT), where the patient is anaesthetised but not given ECT

21
Q

What is an active placebo?

A

drug which has its own inherent effects but none for the condition that it is being given.

This does not mean that it will lack any ACTIVE effects

e.g. using atropine for a study into antidepressants

22
Q

What is a nocebo?

A

a placebo that produces prominent side effects

23
Q

In which types of studies, are placebo effects seen to be more powerful?

A

Treatments that are perceived as being more powerful tend to have a stronger placebo effect than those that are perceived to be less so

e.g. injected placebos have a greater effect than oral placebos

24
Q

What does ‘placebo sag’ mean?

A

a situation where the placebo effect is diminished (attenuated) with repeated use

25
Q

In what kind of illness is the placebo effect greater?

A

in mild illness

26
Q

What is the duration of the placebo response?

A

usually quite short lived

27
Q

What does ‘regression to the mean’ refer to?

A

The observation that symptoms which are extreme on first measurement are likely to have improved on subsequent measurements

i.e. a variable is extreme on its first measurement, it will tend to be closer to the average on its second measurement.

28
Q

What does SENSITIVITY mean in stats?

A

Proportion of patients with the condition who have a POSITIVE test result

29
Q

What does SPECIFICITY mean in stats?

A

Proportion of patients without the condition who have a NEGATIVE test result

30
Q

What is the positive predictive value?

A

The chance that the patient has the condition if the diagnostic test is positive

31
Q

What is the negative predictive value?

A

The chance that the patient does not have the condition if the diagnostic test is negative

32
Q

What is the consideration for predictive values?

A

They are prevalence dependent

33
Q

What is PRECISION?

A

Quantifies a tests ability to produce the same measurements with repeated tests

34
Q

What is a type 1 error?

A

the null hypothesis is rejected when it is true

35
Q

What is a type 2 error?

A

the null hypothesis is accepted when it is false

36
Q

What is defined as the POWER of a study?

A

probability of (correctly) rejecting the null hypothesis when it is false

  • power = 1 - the probability of a type II error
  • power can be increased by increasing the sample size
37
Q

What is the null hypothesis?

A

states that two treatments are equally effective

A significance test uses the sample data to assess how likely the null hypothesis is to be correct

38
Q

Which are the stable characteristics for a disease or study?

A
  • Precision
  • sensitivity
  • accuracy
  • specificity
39
Q

Why is the positive predictive value dependent on disease prevalence?

A

The positive predictive value is low in conditions whereby the prevalence is low

This is due to the fact that as the prevalence falls, the number of true positives falls also.

1st term: CPH/PPD (4 decks)

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