Stats/Epidemiology Flashcards
What is prevalence?
- The proportion of sampled individuals affected AT A GIVEN POINT IN TIME
- Includes both old and new cases
- Typically expressed as a percent
What is incidence?
- The number of NEW cases occurring in a specified population over a SPECIFIC PERIOD OF TIME
- Typically expressed as cases per individual per unit of time
What is the equation for prevalence in terms of a 2x2 table?
Prevalence = (a+c)/(a+b+c+d)
Prevalence = (animals with the disease)/(total population)
What is sensitivity?
- The likelihood of a POSITIVE test in individuals known to HAVE the disease
- The TRUE POSITIVE rate
What is specificity?
- The likelihood of a NEGATIVE test in individuals that DO NOT HAVE the disease
- The TRUE NEGATIVE rate
What represents the TRUE POSITIVE rate?
Sensitivity
What represents the TRUE NEGATIVE rate?
Specificity
What is the equation for sensitivity in terms of a 2x2 table?
Sensitivity = a/(a+c)
Sensitivity = true positives/everyone that has the disease
What is the equation for specificity in terms of a 2x2 table?
Specificity = d/(b+d)
Specificity = true negatives/everyone that does not have the disease
What is the positive predictive value?
- The likelihood of a POSITIVE test in an animal truly AFFECTED by the disease
- INCREASES as PREVALENCE of disease INCREASES
What is the effect on the positive predictive value as the prevalence increases?
The positive predictive value will increase
What is the negative predictive value?
- The likelihood of a NEGATIVE test in an animal truly NEGATIVE for the disease
- Decreases as PREVALENCE of disease INCREASES
What is the effect on the negative predictive value as prevalence increases?
The negative predictive value will decrease
What is the equation for positive predictive value in terms of a 2x2 table
PPV = a/(a+b)
PPV = true positive/all animals testing positive
What is the equation for negative predictive value in terms of a 2x2 table?
NPV = d/(c+d)
NPV = true negatives/all animals testing negative
What is relative risk?
- The ratio of risk in individuals exposed vs not exposed to the factor
- Also called risk ratio
What is the equation for relative risk in terms of a 2x2 table?
Relative risk = [a/(a+b)]/[c/(c+d)]
Relative risk = (risk in exposed individuals)/(risk in unexposed individuals)
Interpret:
Risk ratio = 1
There is no difference in risk of a given outcome between individuals exposed and unexposed to the risk factor/intervention
Interpret:
Risk ratio < 1
There is less risk of the outcome in individuals exposed to the risk factor/intervention than those unexposed
*The intervention is protective
Interpret:
Risk ratio > 1
There is more risk of the outcome in individuals exposed to the risk factor/intervention
What is the odds ratio?
- The probability of an event happening vs not happening
When is it useful to use an odds ratio?
- Useful to measure associations when INCIDENCE cannot be calculated (ie case control studies)
- Useful when frequency of outcome is LOW (ie would take 80 years to get a large enough population)
What is the equation for the odds ratio in terms of a 2x2 table?
Odds ratio = (a x d)/(b x c)
Odds ratio = (probability that case was exposed)/(probability that a control was exposed)
Interpret:
Odds ratio = 1
There is no association between the factor and the outcome
Interpret:
Odds ratio < 1
There is a negative association between the factor and the outcome
Interpret:
Odds ratio > 1
There is a positive association between the factor and the outcome
What is the likelihood ratio?
use the sensitivity and specificity of the test to determine whether a test result usefully changes the probability that a condition (such as a disease state) exists
What is the equation for the likelihood ratio in a positive test result?
Likelihood Ratio (+) = Sens/(1-spec)
What is the equation for the likelihood ratio in a negative test result?
Likelihood Ratio (-) = (1-sens)/Spec
Interpret:
Likelihood Ratio = 1
The test is useless
What would be a good likelihood ratio for a positive result?
LR (+) > 10
What would be a good likelihood ratio for a negative result?
LR (-) < 0.1
What is a type 1 error?
- False positive rate (p-value)
- Rejection of a true null-hypothesis
- Ex: An innocent person is convicted
What is a type 2 error?
- False negative rate
- Non-rejection of a false null hypothesis
- Ex: A guilty person is not convicted
With a cohort study, the (relative risk/odds ratio) should be used?
Relative risk
With a case control study, the (relative risk/odds ratio) should be used
Odds ratio
A cohort study can be (Prospective/Retrospective/Both)
Both
A case control study can be (prospective/retrospective/both)
Retrospective
What does incomplete penetrance mean in terms of inheritance patterns?
The person either expresses the disease phenotype or they do not when they have the disease
Incomplete penetrance is associated only with (dominant/recessive) inheritance, never (dominant/recessive) inheritance
- Dominant
2. Recessive
A high prevalence would be indicative of a(n) (acute/chronic) disease
Chronic
If the incidence is higher than the prevalence, it would be indicative of a(n) (acute/chronic) disease
Acute
Disinfectants that work in the presence of organic matter
- Aldehydes
- Phenols
- Quaternary ammonium compounds (roccal)
