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Biostatistics

1. Incidence = # of new casesa. Also equals ABSOLUTE risk

2. Prevalence = total number of cases that exist (old or new)a. Prevalence>incidence in chronic diseases

3. Sensitivitya.

A/ A + Cb. Percent of patients with disease that will have a positive screening test

c. Highly sensitive test reduces probability of missing diagnosisd. Negative on highly sensitive test rules disease oute. Highly sensitive tests can have high false positive rates (why you also need a highly specific

test)

4. Specificitya. D / D + Bb. Percentage of normal patients who have a negative screening testc. Highly specific test that is positive rules in the diagnosis

5. Positive Predictive Valuea.

A / A + B

b. Chance of disease given a positive or negative test resultc. Varies with populationd. If prevalence of diseases decreases, then PPV decreases and NPV increases

6. Negative Predictive Valuea. D / C + Db. Varies with population/prevalence of diseasec. An overly sensitive test w/ lots of false positives will increase NPV

7. Likelihood Ratioa. Proportion of patients w/ disease who test positive / proportion of patients w/o disease

who test positive

b. 2, 5, 10 increase probability by 15%, 30% and 45% respectivity8. Post-test probabilitya. Sensitivity / (False + Rate)

9. Odds Ratio (aka RISK)a. (A x D)/(B x C)b. Used only for RETROSPECTIVE (Case-controlled) studiesc. Compares disease in exposed & nondisease in unexposed

10.Relative Riska. [A/(A+ B)] / [C/(C+D)]b. Can only be calculated after prospective or experimental studies.c. RR > 1 is clinically significant

11.Attributable Risk Percent (ARP)a. Represents the excess risk in a population that can be explained by exposure to a particular

risk factor.

b. ARP = (risk in exposed risk in unexposed)/Risk in exposedc. ARP = (RR -1) /RRd. [A/(A+ B)] - [C/(C+D)]e.

Relative risk is used in cohort studies to determine how strongly the outcome can be attributed to the risk. Null value RR is 1.0

RR > 1 = there ia an association between the exposure & the disease -p value & confidence intervals help strengthen the

findings

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Cross-sectional study = prevalence study - characterized by simultaneous measurement of exposure & outcome - used for

surveys commonly

PPV = TP/(TP + FP)

Lowering cutoff point for a test will INCREASE SENSITIVITY (fewer false negatives) and decrease specificity (more false

positives). Screening tests need HIGH SENSITIVITY. Confirmatory tests need high SPECIFICITY.

CONFOUNDING VS. EFFECT MODIFICAITON

Effect modification results when an external variable positively ornegatively impacts the effect of a risk factor on the disease

of interest. It can be distinguished from confounding by perfeorming a stratified analysis centered on the variable of interest.

Normal distribution---measures of central tendency are equal MEAN = MEDIAN = MODE

Two-sample t test is used to compare the means between 2 groups of subjects.

Randomization is a method to control confounders

Attributable risk percent = (risk exposed - risk in unexposed)/ risk in exposed ARP = (RR-1)/RR

Incidence: measure of new cases Prevalence: measure of those with the disease in the population at a particular time. Prevalence = Incidence x time Increased prevalence & stable incidence can be due to improved quality of care

Loss to follow up ===selection bias

Lead time bias occurs when 2 interventions are compared to diagnose a disease, one intervention diagnoses thedisease eaerlier than the other w/o an effect on the outcome

Measurment bias = occurs from poor data collection w/ inaccurate results Recall bias = participant is affected by prior knowledge, occurs in case-control studies Susceptibility bias = selsection bias where a treatment regimen is selected for a patient based on the severity of their

condition, w/o taking into account confounding variables

NPV will vary with the pretest probability of the disease.

The prevalence of a disease is directly related to the pre-test probability of having the disease & also affects the NPV.

False positive ratio = 1-specificity False negative ratio = 1-sensitivity

Prospective cohort studies are best to determine the incidence of a disease.NNT = 1/ARR

Case-controlled studies are popular to study exposure-disease association (cheap & less timeconsuming than cohort studies)

o Drawback: Risk can not be derived directly from resultso Exposure-odds ratio is the measure of association in case-controlled studies.o Relative risk can NOT be calculated from case-control studies. But may be approx. equal to

the odds ration.

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o The outcome of a case-control study is not common in the population, the odds ratio isclose to the relative risk. \

o Move cutoff point from A B: SPECIFICITY INCREASED (fewer false positives)o Move cutoff point from BA: Increase FP & decrease FN ( specificity & sensitivity)

Raise cut off value specificity (fewer FP) & sensitivity (more FN) Screening tests need high sensitivity Confirmatory tests need high specificity