Biostatistics Flashcards
Define sensitivity.
The sensitivity of a test refers to how often a positive result correctly identifies those who have the disease. The higher the sensitivity, the lower the number of results that are falsely negative.
How is sensitivity calculated?
Sensitivity = true positive/(true positive + false negative) = true positive / total diseased
How can SPIN and SNOUT be used to help differentiate between sensitivity and specificity?
SPIN: SPecific tests help to rule IN a disease because the false Positive rate is low. SNOUT: SeNsitive tests help to rule OUT a disease because the false Negative rate is low.
Define specificity.
The specificity of a test determines how often a negative result correctly identifies those who do not have the disease. The higher the specificity, the lower the number of false positive test results (i.e. higher specificity = higher likelihood that a negative result indicates true absence of disease).
How is specificity calculated?
Specificity = true negative / (true negative + false positive) = true negative / total not diseased
Draw a Bayesian foursquare (test results and disease statuses) and use it to demonstrate how to calculate sensitivity.
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Draw a Bayesian foursquare (test results and disease statuses) and use it to demonstrate how to calculate specificity.
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Draw a Bayesian foursquare (test results and disease statuses) and use it to demonstrate how to calculate positive predictive value.
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Draw a Bayesian foursquare (test results and disease statuses) and use it to demonstrate how to calculate negative predictive value.
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Define positive predictive value.
The positive predictive value of a diagnostic test is the probability that a patient with a positive test actually has the disease being tested for.
How is positive predictive value calculated?
PPV = true positive / (true positive + false positive) = true positive / all positive tests
What does the positive predictive value help determine?
PPV helps to determine how effective a test is as a screening tool.
Describe the relationship between prevalence of a disease and its effect on sensitivity/specificity vs its effect on positive and negative predictive values.
Sensitivity and specificity are independent of disease prevalence, whereas positive and negative predictive values are influenced by the prevalence of a disease.
Define negative predictive value.
Negative predictive value of a diagnostic test is the probability of not having a disease if the test is negative.
How is negative predictive value calculated?
NPV = true negative / (true negative + false negative) = true negative / all negative tests
Define prevalence.
Prevalence of a disease is the percent of those with the disease in the population being studied.
How is prevalence calculated?
Prevalence = total diseased / total population = (TP + FN) / (TP + FN + TN + FP). ***Note that the Bayesian formula is the same for both prevalence and incidence, but the study setup is different.
How does prevalence affect how one might interpret a given test result?
Prevalence affects the pretest probability associated with a given test.
Define incidence.
The incidence of a disease is the occurrence of new cases of a disease within a specified period of time (it can also be seen as the probability that a person develops that disease during the period of time).
How is incidence calculated?
Incidence = total new cases of disease / total # tested during the given time period = (TP + FN) / (TP + FN + TN + FP). ***Note that the Bayesian formula is the same for both prevalence and incidence, but the study setup is different.
What happens if you increase the threshold of a test?
If you increase the threshold of a test you get more negative results (both true and false negatives). This decreases the sensitivity and increases the specificity.
How can one conceptualize the threshold of a test in relationship with the Bayesian foursquare?
Think of the threshold like the horizontal line of the Bayesian foursquare: as the line moves upward (threshold increases), the total number of negative tests increases; as the line moves downward (threshold decreases), the total number of negative tests decreases. These changes will then be reflected in the sensitivity and specificity calculations due to their affects on the numerators and denominators of those equations.
How is sensitivity affected by changes in the threshold of a test?
As the threshold increases, the sensitivity decreases.
How is specificity affected by changes in the threshold of a test?
As the threshold increases, the specificity increases.