Statistics + Epidemiology Flashcards
Recall 2 x 2 table
- PPV
- NPV
- Specificity
- Sensitivity
- Pos likelihood ratio
- Neg likelihood ratio
2x2 table. How to calculate:
-RR
- OR
- NNT
- NNH
What is positive predictive value and how to calculate?
Positive predictive value
* Predictive value of test
* Chance if the est is positive of patient actually having the disease
- precision of test
* PPV = True positives / all the positives
PPV = T+ / (T+ + F+)
What is negative predictive value and how to calculate?
Negative predictive value
* If the test is negative, what is the chance of patient actually not having the disease
* NPV = True negative outcome/ over all negative outcomes
NPV = T- / (T- + F-)
Sensitivity - what, how to calculate, when most useful?
Sensitivity
* True positive rate of test
* Out of all people with condition, how many does test report is positive?
* Only concentrating on people with the condition (Condition +)
* Sensitivity = True positives / (true positives + false negatives)
* Test has high sensitivity - reports or overreports condition, unlikely to miss condition
Test has sensitivity, negative result is useful for ruling out disease
Specificity - what, how to calculate, when most useful?
Specificity
* True negative rate
* Of all the people who don’t have condition, how many does the test report as negative
* Specificity = true negatives / (true negatives + false positives)
Test with high specificity, if have positive result useful to rule in disease
Calculate positive predictive value from Sens/Spec/Prevalence
- steps involved
1) True positives
2) False positives
3) Positive predictive value
Likelihood ratio + - what, how to calculate
- Assess value of performing a diagnostic test
- Uses sensitivity and specificity of test to determine whether a test result usefully changes the probability that a condition exists
- Probability of a true positive (with disease) to false positives (without disease)
- LR = sensitivity/ (100% - specificity)
Positive likelihood ratio - larger big number, better, increases post test probability - worthwhile testLikelihood ratio of a positive test
- LR = sensitivity/ (100% - specificity)
Likelihood ratio of negative test - what, how to calculate?
Likelihood ratio of negative test
* Probability of false negative (with disease) to a true negative (without disease)
LR - = (100% - sensitivity)/ specificity
Negative likelihood ratio - lower closer to 0, better, decreases post test probability, helpful to rule out - useful test
Pre-test probability - what, how to calculate
Pre-test probability
* Probability of patient having disease before diagnostic test is known
* Essentially prevalence within subject population
Pre-test probability = proportion of patients with disease/all patients with the symptoms (with and without disorder)
Bias - confounding, ecological fallacy, selection, recall?
· Confounding - distortion in measure of risk factor and outcome by mixing effect with another exposure
· Ecological fallacy - not applying clinical decision instrument to patient in front of you
· Selection bias - group selected for study not representative wider population
Recall bias - do not correctly remember events in past
What is incidence?
- Rate
Eg cases per 5 years
What is prevalence?
- “Point”/ cross section in time
- Point prevalence studies
Prevalence = Incidence x mean survival
- Point prevalence studies
Screening tests - what do you want?
- High sensitivity
- High pos likelihood ratio - bigger number, increase post test probability
Pos predictive value
- High pos likelihood ratio - bigger number, increase post test probability
WHO guidelines for screening? (Wilson’s criteria)
- Important health problem
- Need treatment
- Facilities to diagnosis and treatment
- Latent stage
- Test or examination
- Test acceptable to population
- Natural history - adequately understand
- Agreed policy on whom to treat
- Cost, economical
Continuous process
