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43 Cards in this Set
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Define Incidence

# new cases of a disease that occur during a period of time
in a population at risk for developing the disease. 

Define Prevalence

Total # diseased people
 Total Population 

Define Probability

A number expressing the likelihood that a specific event will occur.


How is Probability expressed?

As the ratio of:
# of Actual Occurrences  # of Posssible Occurrences 

What is an alternate representation of Probability?

Risk


What is Odds?

The probability of an event divided by the probability of a nonevent


Formula for Odds:

p
 1p 

What is an odds ratio?

A ratio of two odds


If there's an 80% chance of rain today
And a 20% chance of rain tomorrow What is the odds ratio for it raining today? 
Today = .8 = 4:1
Tomor = .2 = 1:4 .8 / .2 = 16 

If 90% of men have drunk beer in the previous week, what is the odds of men drinking beer?

90:10 = 9:1


If 20% of women have drunk beer in the previous week, what is the odds of women drinking beer?

20:80 = 1/4 = 0.25:1


What is the odds ratio for men drinking beer?

9:1/1:4 = 36


What is the danger of using an odds ratio?

It can overstate the actual reality  36 means men are more likely to have drunk beer, but actually they are only 4.5 times more likely.


Define sensitivity:

How often a test detects a disease when the disease is present.


A 100% sensitive test has:

no false negatives


Define Specificity:

How often a test excludes a disease that is absent.


A 100% specific test has:

No false positives; if it's positive, it's positive, because ALL NEGATIVES will for sure be undiseased.


Positive predictive value:
(formula) 
Probability that someone with a pos test has the disease.
TP/TP+FP 

Negative predictive value:

Probability that someone with a neg test DOESn't have the disease.
TN/TN+FN 

Define Likelihood ratio

How many times more or less likely a test result is found in diseased vs healthy individuals.


Formula for LR:

Sensitivity
 1Specificity 

What type of study is a clinical trial? (2 features)

Experimental
Prospective 

Purpose of a Clinical Trial:

To answer a focused question, usually comparing 2 or more treatment alternatives.


3 types of observational studies:

Cohort
Casecontrol Crosssectional 

When are observational studies used?

When it is impractical or unethical to conduct a clinical trial.


What is a Cohort study?

A study of 2/more groups:
one is exposed to a risk factor other is control 

What is the timeframe of a cohort study like?

Prospective; starts at time=0 and works forward from there.


Are cohort studies always prospective?

No; they can be retrospective.


How is a Retrospective Cohort study done?

Identify cohort that was exposed in the past
Compare to control group See who has outcome now 

What is the usual measure of the outcome in a cohort study?

Incidence


Advantage of cohort studies:

Can look for multiple diseases as a result of one exposure.


What is the timeframe of a Case Control study?

Retrospective  starts with the outcome and works back in time.


What are you looking back in time for in a case control study?

Possible exposures or risk factors.


What is the outcome measure for a Case control study? Why?

Odds ratio  because you can't calculate incidence.


Advantage of casecontrol studies:

Can look for multiple diseases as a result of one exposure.


What type of curve is used for survival analysis?

Kaplanmeier


What is Survival Analysis used for?

For timetoevent data
To follow treatment of terminal diseases when subjects are enrolled for different lengths of time 

4 examples of TimetoEvent data:

Time to cure
Time to return to full function Time to death Time to relapse 

What is the curve that is used for survival analysis?

Kaplanmeier survival curve


What is the fact that the kaplanmeier curve is based on?

The fact that the probability of multiple independent events is the product of the probabilities of each event


How is a survival analysis calculated?

By multiplying the survival percentages at the beginning of each interval.


How does censored data affect survival analysis?

Censored data are subtracted from both the numerator and denominator


How do you see if there is a significant difference between 2 survival curves?

By using a logrank test to generate a p value.
