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Epi > Terms > Flashcards

Terms Flashcards

1
Q

Name 3 observational studies? Are they longitudinal

A
  1. Case series (NL)
  2. Ecological (NL)
  3. Cross sectional (NL)
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2
Q

What is the purpose of a cross sectional study?

Advantages and Disadvantages

A

1 question at 1 point in time
Ad:
Cheap and easy way to explore associations
Disad:
Weak evidence of causality as they lack temporality

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3
Q

What is a case control study?

What are ads and disads

A

It is a retrospective study, i.e. it finds people with an outcome first and then matches them to controls to determine associations between exposures and outcomes.
Ads:
Good for rare conditions, cheap and easy, dont need to follow people over time
Disads:
Recall bias, weak temporal relationship, association doesnt equal causation

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4
Q

Cohort study

ads and disads

A

Find people with exposure and observe if they develop an outcome as compared to controls (i.e. who did not have the exposure).
Ad:
good temporal info, can look at multiple exposures
disad:
Bad for rare conditions, need to follow people over time, tough to determine explicit link b/w exposure and outcome

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5
Q

Interventional clinical trial

A

Give ppl drugs or something to alter circumstances and compare to control.
It is super expensive but gives best information about effectiveness of intervention.

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6
Q

What is bias?

A

It is an unintentional systematic error which leads to differences between groups

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7
Q

Two types of bias?

A

Selection bias:
- Misrepresent population (i.e. do not have an appropriate cohort represented)
Information bias:
- Method of collection of information is biased
- Recall bias (memory of participant)
- Measurement bias (poor equipment i.e. bp machine)

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8
Q

What is confounding?

A

This is where one fails to realise that there is a 3rd variable which is affecting the relationship between exposure and outcome. For example a link between baldness and CVD (the confounders are age or gender)

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9
Q

How do you protect against confounding?

A

Randomization

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10
Q

How do you protect against bias?

A

Blinding (preferably double or even triple blinding)

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11
Q

What is the intention to treat analysis and when is it used?

A

Treat everyone as if they stayed in there originally assigned group. There is a tendency for those in the intervention group to move to the control and those in the control to move to the intervention. The ITT essentially reduces the potential to find significance which is helpful in that stronger relationships between treatment and outcome need to be present in order to derive a positive result.

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12
Q

Prevalence

A

The total number of people at a given time with the condition/disease/thing of interest

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13
Q

Prevalence

A

The total number of people at a given time with the condition/disease/thing of interest - expressed as a percentage

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14
Q

Incidence

A

The probability of occurrence of a given medical condition in a population within a specified period of time

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15
Q

Absolute risk

A

Probability of a disease occurring in a disease free population during a specified period of time.
eg number of cases/number of people in the study

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16
Q

What sort of study is needed in order to establish absolute risk or absolute rate?

A

Longitudinal

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17
Q

Absolute rate

A

Probability of a disease occurring in a disease free population during the sum of individual follow up periods.

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18
Q

Which is better absolute risk or absolute rate?

A

Absolute rate is better because it takes into account the fact that some people may not have remained in the study throughout its duration.

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19
Q

Hazard

A

This is a special kind of instantaneous rate which is measured during close follow up. it may be calculated as follows:
Outcomes/subjects (per week for example)

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20
Q

What are the limitations of absolute risk, absolute rate and hazard?

A

They give us no information about association with exposure. Whilst relative risk and attributable risk do.

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21
Q

What is relative risk?

A

Relative risk (RR) is the ratio of the probability of an event occurring (for example, developing a disease, being injured) in an exposed group to the probability of the event occurring in a comparison, non-exposed group. Relative risk includes two important features: (i) a comparison of risk between two “exposures” puts risks in context, and (ii) “exposure” is ensured by having proper denominators for each group representing the exposure

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22
Q

How is relative risk calculated?

A 
For example where the probability of developing lung cancer among smokers was 20% and among non-smokers 1%. 
Risk	Disease status
                Present	Absent
Smoker	        a	b
Non-smoker	c	d

Here, a = 20, b = 80, c = 1, and d = 99. Then the relative risk of cancer associated with smoking would be
RR=Risk(exposed)/Risk(unexposed)
= {a/(a+b)}/{c/(c+d)} = {20/100}/{1/100} = 20
Smokers would be twenty times as likely as non-smokers to develop lung cancer.

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23
Q

What studies enable the calculation of relative risk?

A

Cohort studies

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24
Q

What is the attributable risk?

A

Attributable risk is the difference in rate of a condition between an exposed population and an unexposed population.

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25
Q

How do you calculate attributable risk?

A 
For example where the probability of developing lung cancer among smokers was 20% and among non-smokers 1%. 
Risk	Disease status
                Present	Absent
Smoker	        a	b
Non-smoker	c	d

Here, a = 20, b = 80, c = 1, and d = 99. Then the relative risk of cancer associated with smoking would be
AR= Risk(exposed) - Risk(unexposed)
= a/(a+b} -c/(c+d) = 20/100 - 1/100
= 19/100p-yr

26
Q

What studies enable the calculation of attributable risk?

A

Mostly calculated in cohort studies

27
Q

What is the attributable risk %?

A

Proportion of incidence of disease among exposed people that is due to exposure

28
Q

How is attributable risk % calculated ?

A

• PAR% = [(Rt - Ru) / Rt] x 100
• example:
Rt=8/100p-yr; Ru=5/100p-yr; PAR=3/100p-yr
PAR% = [(8-5) ÷ 8 ] x 100 = 38%

29
Q

How is population attributable risk calculated ?

A

PAR = Rt* - Ru
• eg: Rt=8/100p-yr; Ru=5/100p-yr; PAR=3/100p-yr
• indicates the additional or excess risk/rate of the
outcome in the population, due to the exposure

*Rt=risk/rate in whole population (both exposed and unexposed)

30
Q

How is population attributable risk% calculated ?

A

• PAR% = [(Rt - Ru) / Rt] x 100
• example:
Rt=8/100p-yr; Ru=5/100p-yr; PAR=3/100p-yr
PAR% = [(8-5) ÷ 8 ] x 100 = 38%

31
Q

How is population attributable risk calculated ?

A 
For example where the probability of developing lung cancer among smokers was 20% and among non-smokers 1%. 
Risk	Disease status
                Present	Absent
Smoker	        a	b
Non-smoker	c	d

Here, a = 20, b = 80, c = 1, and d = 99. Then the relative risk of cancer associated with smoking would be
PAR = Risk(total) - R(unexposed)
= 21/100 - 2/100
= 19/200

32
Q

How is population attributable risk% calculated ?

A 
For example where the probability of developing lung cancer among smokers was 20% and among non-smokers 1%. 
Risk	Disease status
                Present	Absent
Smoker	        a	b
Non-smoker	c	d

Here, a = 20, b = 80, c = 1, and d = 99. Then the relative risk of cancer associated with smoking would be
PAR% = [{Risk(total) - R(unexposed)}/Risk(total)] x 100
= [(a+c)/(b+d) - (c/c+d)]/{(a+c)/(b+d)}]x100
= [(21/200 - 2/200)/(21/200)] x 100
= 90.5%

33
Q

What is population attributable risk?

A

Indicates the additional or excess risk/rate of the outcome in the population due to exposure.

34
Q

What is population attributable risk%?

A

Proportion of disease amongst the whole population that is due to exposure.

35
Q

What is the odds ratio?

A

Approximation of the relative risk conferred by exposure in case control study

36
Q

How is the odds ratio calculated?

A

odds of exposure to non-exposure among controls/odds of exposure to non-exposure among cases.

37
Q

What is a survival analysis

A

This is a plot of hazard vs survival.

Following intervention how many die/survive and how long does it take?

38
Q

What is Hazard?

A

The absolute number of deaths per unit time

39
Q

what is the Hazard ratio?

A

It is similar to relative risk and applied to the whole period of follow up in a clinical trial. It is a value between 0 and (as a reduction in outcome) so a HR of 0.6 = a reduction in the outcome for those in the test group of 40%

40
Q

What is the Null Hypothesis?

A

The null hypothesis is the assumption that the results happened due to chance. An interventional study aims to reject the null hypothesis.

41
Q

What is the Confidence interval?

A

The confidence interval is the interval over which we can be 95% sure that the hazard ratio lies within a certain region. If the region includes the null hypothesis (i.e. 1), it means the drug is not effective or could actually be doing the opposite of what was intended.

42
Q

Risk/rate reduction

A

rate of outcome intervention/rate of outcome control

43
Q

Absolute risk/rate reduction

A

rate of outcome control - rate of outcome intervention (measured in person years)

44
Q

What is the Number needed to treat?

A

This is a determination of the number of people needed to undergo the intervention in order to prevent the outcome in just one. It is a marker of the efficacy of the intervention (i.e. a good drug will have a low NNT).

45
Q

How to calculate number needed to treat:

A

1/absolute risk or rate reduction - it is given in years

46
Q

What is the number needed to harm?q

A

When interventions increase risk/rate of outcome (ie if intervention is taking cocaine)
NNT = -25 per year
NNH =25 per year

47
Q

Sensitivity- what is it?

A

What proportion test positive out of those with the disease

48
Q

How to calculate sensativity

A

True positive (TP)/(TP + FN)

49
Q

Specificity- what is it?

A

What proportion of the people test negative of those people without the disease?

50
Q

How to calculate specificity?

A

TN/(TN+FP)

51
Q

Positive predictive value - what is it?

A

What proportion are truly positive of those people who test positive

52
Q

How to calculate Positive predictive value?

A

TP/(TP+FP)

53
Q

Negative predictive value- what is it?

A

What proportion are truly negative out of those people who test negative?

54
Q

How to calculate negative predictive value?

A

TN/(TN+FP)

55
Q

What study allows the determination of the survival analysis?

A

RCT

56
Q

How is hazard calculated?

A

Started with 1000 people, 10 died in week

- hazard for week one = 10/1000

57
Q

What study is used to calculate hazard?

A

RCT

58
Q

How is hazard ratio calculated?

A

Hazard(intervention) : hazard (control)

59
Q

What is the difference between diagnosis and screening?

A

Diagnosis is a confirmation of disease or otherwise a means of establishing the causative agent or problem. Whilst screening is a means of determining those more likely to have a disease

60
Q

What is lead time bias?

A

Lead time is the length of time between the detection of a disease (usually based on new, experimental criteria) and its usual clinical presentation and diagnosis (based on traditional criteria).
Lead time bias is that factor when comparing, in detection of disease, a traditional test with a new or experimental test but the outcome of the disease is unchanged; the new or experimental test merely identifies the disease earlier than the previous and thus gives the impression that survival is prolonged. It is an important factor when evaluating the effectiveness of a specific test.

61
Q

What is length time bias?

A

Length time bias is a form of selection bias, a statistical distortion of results which can lead to incorrect conclusions about the data. Length time bias can occur when the lengths of intervals are analysed by selecting intervals that occupy randomly chosen points in time or space. This process favours longer intervals, thus skewing the data. Length time bias is often discussed in the context of the benefits of cancer screening, where it can lead to the perception that screening leads to better outcomes when in reality it has no effect.

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