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SBM II December > Biostatistics > Flashcards

Biostatistics Flashcards

1
Q

What does Prevalence, Incidence and Attack Rate tell us?

A

What is the frequency of disease in a population?

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2
Q

What does Sensitivity and specificity tell us?

A

How well does a test differentiate sick from healthy people?

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3
Q

What does predictive value tell us?

A

Of those in a population who test as sick of healthy, how many are truly sick or healthy?

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4
Q

What does Risk Reduction/Increase and Number-needed-to-treat/harm tell us?

A

What is the impact of a medicine/treatment?

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5
Q

What does point prevalence help us understand?

A

Disease burden or extent of a health problem.

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6
Q

What is prevalence?

A

[Number with a disease at a specific point in time]/[Number at risk of illness during that time period]

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7
Q

What is period prevalence?

A

Prevalence during a period of time

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8
Q

What is lifetime prevalence?

A

Prevalence over the course of a lifetime

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9
Q

What does incidence help us understand?

A

The risk of a specific health event

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10
Q

What is incidence?

A

[Number of NEW people with DZ during a time period]/[Number at risk of illness during that time period]

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11
Q

What is the main measure of acute diseases?

A

Incidence

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12
Q

What helps determine causation?

A

Incidence

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13
Q

What is cumulative incidence?

A

Total number reported over time

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14
Q

What is Attack Rate?

A

Refers to outbreaks - similar to prevalence over a very short period of time

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15
Q

When is Attack Rate used?

A

When the nature of disease is acute and population observed for short period of time (ex. outbreaks, specific exposures)

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16
Q

How do you calculate Attack Rate?

A

[Number new cases]/[Number exposed]

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17
Q

How do you calculate Secondary Attack Rate?

A

[Number new cases]/[Number exposed - primary cases]

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18
Q

What does Secondary Attack Rate measure?

A

Person to person spread of disease after initial exposure

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19
Q

What is Secondary Attack Rate similar to over a very short period of time?

A

Incidence

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20
Q

What affects prevalence and incidence?

A
  • Duration of illness (longer –> higher prevalence)
  • Number of new cases (more new cases –> higher prevalence) - incidence high
  • Migration - In (ill –> higher prevalence); Out (well –> higher prevalence)
  • ->Recovery and death –> lower prevalence
  • Prevention –> lower incidence
  • Changes in diagnostic criteria or reporting
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21
Q

What is the relationship between prevalence and incidence if the disease is long term (ex. diabetes)?

A

Prevalence > Incidence

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22
Q

What is the relationship between prevalence and incidence if the illness is acute (ex. flu)?

A

Prevalence ~ Incidence

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23
Q

What is sensitivity?

A

The probability that a diseased person will be identified correctly by a diagnostic/screening test

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24
Q

What is another name for sensitivity?

A

True-positiive probability or true-positive rate

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25
Q

What is the equation for sensitivity?

A

True Positives/ Total # of ill people

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26
Q

What should you remember with Sensitivity?

A

SNOUT - High sensitivity rules disease out

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27
Q

What is the total number of ill people?

A

True positives + False negatives

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28
Q

What is Specificity?

A

Probability that a well (non-diseased) person will be identified correctly by a diagnostic/screening test

29
Q

What is another name for specificity?

A

True-negative probability

30
Q

What is the equation for specificity?

A

True negatives/total # of well people

31
Q

What should you remember with Specificity?

A

SPIN - High specificity disease rules in

32
Q

What is the total # of well people?

A

TN + FP

33
Q

What does a high sensitivity test err on the side of?

A

Over-diagnosing

34
Q

What does a high specificity test err on the side of?

A

Under-diagnosing

35
Q

What should you remember with high sensitivity tests?

A
  • Identify most or all possible disease cases; may identify some healthy people as sick
  • Most useful when under-diagnosing may lead to severe consequences (ex. fast developing cancers)
36
Q

What should you remember with high specificity tests?

A
  • Identify most or all well people; may miss some of the sick people
  • Most useful when over-diagnosing leads to dangerous, painful or unnecessary treatment
37
Q

What is a predictive value?

A

Probability that a test will give the correct diagnosis

38
Q

What does predictive value depend on?

A
  • Test sensitivity and specificity; prevalence of the DZ in the population being tested
  • Predictive values will vary from population to population and study to study
39
Q

What is Positive Predictive Value?

A

Probability that a person who tests positive for a disease truly has it (is really sick)

40
Q

What is the equation for PPV?

A

PPV = TP/(TP + FP) –> Top row of a 2x2 table

41
Q

What is the equation for NPV?

A

NPV = NP/(NP + FN) –> Bottom row of a 2x2 table

42
Q

What is Negative Predictive Value?

A

Probability that a person who tests negative for a disease truly is well

43
Q

How does High prevalence relate to predictive value?

A
  • Higher disease prevalence –> Higher PPV (greater chance that positive test result reflects true illness)
  • -> Lower NPV (lower change that negative test reflects disease-free status)
44
Q

How does Low prevalence relate to predictive value?

A
  • Lower disease prevalence –> Lower PPV (lower chance that positive test result reflects true illness)
  • -> Higher NPV (greater chance that negative test result reflects disease-free status)
45
Q

When is Risk Reduction and Number-Needed-To-Treat relevant?

A

When comparing effects in randomized controlled trials.

46
Q

Why are we interested in Risk Reduction and Number-Needed-To-Treat?

A

Interested in understanding risk of treatment vs. no treatment

47
Q

What are we asking in Risk Reduction and Number-Needed-To-Treat studies?

A

What is the frequency of bad outcomes in group being treated compared to the group not being treated?

48
Q

Randomized Controlled Trials (RCT):

A
  • Have at least one treatment group and one control group
  • People in both groups may have positively (placebo effect) or negatively (harmful effects)
  • How do we compare different group response rates?
49
Q

What is Control Event Rate (CER)?

A

Proportion of control group participants who have a bad outcome after “treatment” (ex. placebo or no rx)

50
Q

What is the CER if 10 of 30 control group participants become sicker?

A

CER = 10/30 = 33% have adverse outcomes

51
Q

What is Experimental Event Rate (EER)?

A

Proportion of treatment group participants who have a bad outcome after treatment (ex. new drug)

52
Q

What is the EER if 4 of 30 treatment groups become sicker?

A

EER = 4/30 = 13% had adverse outcomes

53
Q

What is Absolute Risk?

A

“risk difference” = difference in risk of developing a DZ or undesired outcome after treatment

54
Q

How do you calculate Absolute Risk?

A

CER-EER

55
Q

What is an Absolute Risk Reduction (ARR)?

A

When CER > EER - higher rate of adverse outcomes in control group –> sometimes referred to as “attributable risk”

56
Q

What is an Absolute Risk Increase (ARI)?

A

When EER > CER - higher rate of adverse outcomes in treatment group

57
Q

What is Relative Risk?

A

“risk ratio” = proportion of treatment group risk to control group risk

58
Q

How do you calculate Relative Risk?

A

EER/CER

59
Q

How does risk of bad outcome change in the treatment group with RR?

A

Risk Increases when RR > 1

Risk Decreases when RR

60
Q

What is Relative Risk reduction/increase?

A

Difference in 2 event rates, as a proportion of the event rate in the control group

61
Q

What is the equation for Relative Risk Reduction/Increase?

A

1-RR or AR/CER

62
Q

What is the equation for Relative Risk Reduction?

A

CER > EER

63
Q

What is the equation for Relative Risk Increase?

A

EER > CER

64
Q

What is Number Needed To Treat (NNT)?

A

Number of patients who need to be treated to get 1 additional patient a favorable outcome

65
Q

What is the equation for NNT?

A

NNT = 1/ARR

66
Q

Explain what NNT = 5 means?

A

For every 5 people treated, 1 more person would respond to the drug

67
Q

What is Number Needed to Harm (NNH)?

A

Number of patients who, if they were treated, would result in 1 additional patient being harmed

68
Q

How does NNH relate to ARI?

A

NNH = 1/ARI

69
Q

Explain wheat NNH = 3 means?

A

If 3 people were treated, 1 more person would not respond compared with the control group.

SBM II December (11 decks)

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