statistics Flashcards
Decreasing the pre-test probability has what effect on positive predictive value?
Decreases PPV.
Decreasing the pre-test probability has what effect on negative predictive value?
Increased NPV (inversely proportional)
Equation for PPV
True positives/True positives + False positives
TP/TP + FP
Equation for NPV
True negatives/True negatives + False negatives
TN/TN + FN
Equation for sensitivity
True positives/True Positives + False negatives
TP/TP + FN
Equation for specificity
True negatives/true negatives + false positives
TN / TN+FP
What is the usual outcome measure in a cohort study?
Relative risk
What is the usual outcome measure in a case-control study?
odds ratio
Describe a type 1 error
What is it increased by?
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
Describe a type 2 error
What is it decreased by:
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)
Define the power of a study
What is the equation?
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
What is the equation for NNT?
NNT = 1 / Absolute Risk Reduction
What is the equation for absolute risk reduction?
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)
What is the equation for relative risk?
Relative risk = EER / CER
When are odds ratios most commonly used?
Case control studies