statistics Flashcards

1
Q

Decreasing the pre-test probability has what effect on positive predictive value?

A

Decreases PPV.

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2
Q

Decreasing the pre-test probability has what effect on negative predictive value?

A

Increased NPV (inversely proportional)

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3
Q

Equation for PPV

A

True positives/True positives + False positives

TP/TP + FP

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4
Q

Equation for NPV

A

True negatives/True negatives + False negatives

TN/TN + FN

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5
Q

Equation for sensitivity

A

True positives/True Positives + False negatives

TP/TP + FN

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6
Q

Equation for specificity

A

True negatives/true negatives + false positives

TN / TN+FP

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7
Q

What is the usual outcome measure in a cohort study?

A

Relative risk

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8
Q

What is the usual outcome measure in a case-control study?

A

odds ratio

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9
Q

Describe a type 1 error

What is it increased by?

A

rejecting the null hypothesis when it is true i.e. false positive

“alpha” error

increased if the number of end-points are increased. (i.e. likely that by chance one of these end points will be reached) and increading significance level

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10
Q

Describe a type 2 error

What is it decreased by:

A

accepting the null hypothesis when it is false i.e. false negative

“beta” error

Decreased risk by increasing power and sample size (increased sample size = increased power)

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11
Q

Define the power of a study

What is the equation?

A

Probability of correctly rejecting the null hypothesis when it is false, i.e. the probability of detecting a statistically significant difference

Power = 1 – the probability of a type II error

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12
Q

What is the equation for NNT?

A

NNT = 1 / Absolute Risk Reduction

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13
Q

What is the equation for absolute risk reduction?

A

If the outcome of the study is undesirable
- Control event rate - experimental event rate
i.e. (b/b+d) - (a/a+c)

If the outcome of the study is desirable
- EER - CER
i.e. (a/a+c) - (b/b+d)

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14
Q

What is the equation for relative risk?

A

Relative risk = EER / CER

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15
Q

When are odds ratios most commonly used?

A

Case control studies

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16
Q

What is the equation for relative risk reduction?

A

Experimental event rate - control event rate/control event rate

(EER – CER) / CER

17
Q

What is the equation for likelihood ratio of a positive result?

A

sensitivity/(1 - specificity)

18
Q

What is the equation for likelihood ratio of a negative result?

A

(1 - sensitivity)/specificity

19
Q

Which remains constant across different populations with different risk of underling disease - absolute or relative risk reduction?

A

Relative risk reduction remains constant (ratio)
Absolute risk reduction varies with underlying risk of disease in population

20
Q

What level of evidence do case studies provide?

A

LEVEL III

Ia- evidence from meta-analysis of randomised controlled trials

Ib- evidence from at least one randomised controlled trial

IIa- evidence from at least one well designed controlled trial which is not randomised.

IIb- evidence from at least one well designed experimental trial

III- evidence from case, correlation and comparative studies.

IV- evidence from a panel of experts.

21
Q

What are grade B recommendations based on?

A

Evidence from non-RCT (IIa,IIb,III)

Grade A- based on evidence from at least once RCT (Ia,Ib)

Grade C- based on evidence from a panel of experts (IV).