Biostats 3- Analysis of Screening Tests Flashcards
What is the eqn for a sample size needed for comparing differences in means between 2 groups with an alpha = 0.05?
n = 16s^2/d^2
s= standard deviation d= difference btwn the means
True or False: you use the same eqn for difference among more than 2 means.
True
What is the eqn needed for doing a correlation between 2 continuous variables?
n = 4 + 8/r
r= estimated correlation coefficient
What does “ballparkness” give you?
the order of magnitude of people for a study (multiple of 10)
How much should u multiply your calculated sample size by to reduce migration bias?
1.25
attrition
Why is screening a prevetnative intervention?
cuz u do it on asymptomatic individuals that allows for early detection, therapy, and ↓ mortality from the disease.
What are the 4 selection criteria for a disease that can be screened?
- Disease should be an important public health problem
- Early detection in asymptomatic individuals is possible and improves outcome
- Screenign tests are acceptable/inexpensive/accurate
- Treatment is available for disease
For Bayes Theorem, what is the tool you use to describe screening test outcomes within a popualtion?
2x2 contigency table
This is a proportion of true positives correctly identified by the test over those wwho do have the disease
(test + | pt present)
Sensitivity
What is the SE eqn for sensitivity?
SE = a/(a + c)
What is the opposite of sensitivity, where its the proportion of true negatives over those who DONT have the disease?
(test - | no disease)
Specificity
What is the eqn for specificity?
Specificity = d/(b + d)
True or False: the higher the sensitivity, the fewer real cases of diseases go undetected.
True
True or False: True higher the specificity, the fewer cases of normal patients go undetected .
True
This is the # diseased per 1,000.
Prevalence
This is the probability that pts with a + test result were correctly diagnosed as having the disease.
Positive Predictive Value (PPV)
What is the eqn for PPV?
PPV = a/(a+ b)
AKA”everyone who tested +”
use rows
This is the proportion of pts with a negative test result that did in fact NOT have the disease.
Negative Predictive Value (NPV)
What is the eqn for NPV?
d/(c + d)
notice we use d here cuz d is the true negative.
Accoring to beyes theorem, waht happens to PPV and NPV as the prevalence of the disease increases?
PPV ↑
NPV ↓
This is a measure of how likely it is that an event will occur.
Probability (P)
In mutually exclusive probabilities, how do you do measure probabilities?
Additive rule
roll a 2 OR a 5…
If 2 events are independent, how do u measure probabilities?
Multiplication rule
roll a 6 and THEN a 5…
What is the % chance a chem-6 panel comes up normal for everything, if each variable has a P=0.95?
74%
