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SBM II December > Biostatistics > Flashcards

Flashcards in Biostatistics Deck (69):
1

What does Prevalence, Incidence and Attack Rate tell us?

What is the frequency of disease in a population?

2

What does Sensitivity and specificity tell us?

How well does a test differentiate sick from healthy people?

3

What does predictive value tell us?

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

4

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

What is the impact of a medicine/treatment?

5

What does point prevalence help us understand?

Disease burden or extent of a health problem.

6

What is prevalence?

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

7

What is period prevalence?

Prevalence during a period of time

8

What is lifetime prevalence?

Prevalence over the course of a lifetime

9

What does incidence help us understand?

The risk of a specific health event

10

What is incidence?

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

11

What is the main measure of acute diseases?

Incidence

12

What helps determine causation?

Incidence

13

What is cumulative incidence?

Total number reported over time

14

What is Attack Rate?

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

15

When is Attack Rate used?

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

16

How do you calculate Attack Rate?

[Number new cases]/[Number exposed]

17

How do you calculate Secondary Attack Rate?

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

18

What does Secondary Attack Rate measure?

Person to person spread of disease after initial exposure

19

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

Incidence

20

What affects prevalence and incidence?

-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

21

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

Prevalence > Incidence

22

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

Prevalence ~ Incidence

23

What is sensitivity?

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

24

What is another name for sensitivity?

True-positiive probability or true-positive rate

25

What is the equation for sensitivity?

True Positives/ Total # of ill people

26

What should you remember with Sensitivity?

SNOUT - High sensitivity rules disease out

27

What is the total number of ill people?

True positives + False negatives

28

What is Specificity?

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

29

What is another name for specificity?

True-negative probability

30

What is the equation for specificity?

True negatives/total # of well people

31

What should you remember with Specificity?

SPIN - High specificity disease rules in

32

What is the total # of well people?

TN + FP

33

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

Over-diagnosing

34

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

Under-diagnosing

35

What should you remember with high sensitivity tests?

-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

What should you remember with high specificity tests?

-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

What is a predictive value?

Probability that a test will give the correct diagnosis

38

What does predictive value depend on?

-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

What is Positive Predictive Value?

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

40

What is the equation for PPV?

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

41

What is the equation for NPV?

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

42

What is Negative Predictive Value?

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

43

How does High prevalence relate to predictive value?

-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

How does Low prevalence relate to predictive value?

-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

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

When comparing effects in randomized controlled trials.

46

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

Interested in understanding risk of treatment vs. no treatment

47

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

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

48

Randomized Controlled Trials (RCT):

-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

What is Control Event Rate (CER)?

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

50

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

CER = 10/30 = 33% have adverse outcomes

51

What is Experimental Event Rate (EER)?

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

52

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

EER = 4/30 = 13% had adverse outcomes

53

What is Absolute Risk?

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

54

How do you calculate Absolute Risk?

CER-EER

55

What is an Absolute Risk Reduction (ARR)?

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

56

What is an Absolute Risk Increase (ARI)?

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

57

What is Relative Risk?

"risk ratio" = proportion of treatment group risk to control group risk

58

How do you calculate Relative Risk?

EER/CER

59

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

Risk Increases when RR > 1
Risk Decreases when RR

60

What is Relative Risk reduction/increase?

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

61

What is the equation for Relative Risk Reduction/Increase?

1-RR or AR/CER

62

What is the equation for Relative Risk Reduction?

CER > EER

63

What is the equation for Relative Risk Increase?

EER > CER

64

What is Number Needed To Treat (NNT)?

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

65

What is the equation for NNT?

NNT = 1/ARR

66

Explain what NNT = 5 means?

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

67

What is Number Needed to Harm (NNH)?

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

68

How does NNH relate to ARI?

NNH = 1/ARI

69

Explain wheat NNH = 3 means?

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