Biostats Flashcards
What does 100% sensitivity indicate?
Low false negative rate
What is the equation for sensitivity?
TP/(TP+FN)
What is sensitivity used for?
Used for screening in diseases with low prevalence
What is the equation for specificity?
TN/(TN+FP)
What does 100 percent specificity indicate?
A low false positive rate
What is the equation for positive predictive value?
TP/(TP+FP)
What is the positive predictive value?
Probability that person actually has the disease given a positive test result
What is negative predictive value?
The probability that a person is actually drug free given a negative test result
What is the equation of negative PPV?
TN/(FN+TN)
What is the true positive rate?
Sensitivity
What is the true negative rate?
Specificity
What is the equation for calculating incidence?
Number of new cases/number of people in population at risk (don’t already have the disease)
What is the equation for prevalence?
Number of cases in a population/total number of people in the population
What is the equation for odds ratio?
Ad/bc
Used in case-control studies (outcome –> exposure)
What is the equation for relative risk?
A/(a+b)/c/(c+d)
What is relative risk?
Risk among exposed/risk among unexposed
What is the equation for absolute risk?
(A/a+b) - (c/c+d)
(RR-1)/RR
#of adverse events in tx/# of adverse events in placebo
What is the number needed to treat?
The number of patients who need to be treated for 1 patient to benefit
What is the equation for the number needed to treat?
1/absolute risk
1/ (event rate -control) - (event rate - tx)
What is the number needed to harm?
Number of patients who need to be exposed to a risk factor for 1 patient to be harmed
What is the equation for the number needed to harm?
1/attributable risk = 1/(a/a+b) - (c/c+d)= 1/((RR-1)/RR)
What is the equation for event rate?
of subjects per tx/total # of subjects
What is precision?
The consistency, reproduciblity and reliability of a test
What lowers precision?
Random error
What is accuracy of a test?
Validity
Absence of systematic error
What is the equation for SEM?
SD / square root sample size
So as sample size increases, standard error of mean decreases
What is the null hypothesis?
Says there is no difference btween two groups
P = high
What is the p-value?
The likelihood of observing what we did if the null is true.
The lower the p-value the more evidence for the alternative hypo
What is a type I error?
False positive error
Stating there s a difference when there really isn’t.
P-value is judged against this
Ex: convicting an innocent man
What is a type II error?
Stating there isn’t an effect or different when there is one
False negative error
Ex: Setting a guilty man free
What is power?
The probablility of seeing a difference when one is there
Ex: convicting a guilty man
= 1- B
What does power increase with?
Sample size
Expect effect size
Precision of measurement
What is the equation for confidence interval?
Mean - Z(SEM) to mean + Z(SEM)
Z= 1.96 for 95% CI
Z = 2.58 for 99% CI
What is the equation for likelihood ratio?
Sensitivity/1-specifity
The higher the better likelihood the patient has a disease they are being treated for
