Epidemiology and ethics Flashcards
Sensitivity
TP/(TP+FN)
True positve rate, all negatives are TN
To screen disease, if negative, SNOUT
Specificity
TN/(TN+FP)
True negative rate, all positive are TP
To confirm disease, if positive, SPIN
Case-control study
Observation, retrospective
“What happened?”
Uses ODD RATIO (OR)
Cohort study
Observation, either prospective or retrospective
Who will develop disease, or who developed disease?
Uses RELATIVE RISKS (RR)
Cross-section study
Observation
“What’s happening”
Determines disease prevalence (DP)
***association but not necessarily causation
Clinical trials
1: few healthy volunteers: safety, toxicity, PK
2: few diseased pts: efficacy, optimal dosing, adverse effect
3: large number of pts: new tx vs current standard of care
4: postmarketing surveillance: detects rare, long term effect
PPV and NPV
PPV: high pretest prob => high PPV
NPV: high pretest prob => low NPV
Incidence vs. prevalence
Incidence rate: # new cases in a time/ pop at risk during the same time period
Prevalence: # exisiting cases/ pop at risk
Prevalence ~ incidence rate x average disease duration
Prevalence > incidence for chornic dz
Odds ratio (OR)
Used in case controlled (obs/retro)
dz expos/dz unexpo)/(healthy expo/healthy unexpo
Relative risk (RR)
Used in cohort (obs/retro or pros)
(dz/all exposed)/(dz/all unexposed)
If prevalence low, RR~OR
Attributable risk
Diff in risk btwn exposed and unexposed
If lung cancer risk 21% in exposed, 1% in unexposed,
then 20% is ATTRIBUTABLE RISK to smoking
*1/AR= number needed to be exposed (for 1 pt to be harmed)
Absolute risk reduction (ARR)
Absolute reduction in risk due to tx compared to control
8% of placebo vs. 2% vaccinated develop flu = 6%
*1/ARR= number needed to treat (for 1 pt to benefit)
Precision vs. accuracy
Precision: consistent and reproducible/reliable, absence f random variation in the test
High precision=> low SD
Accuracy: trueness of test measure, validity, absences of systematic erros or bias in a test
Biases…
Selection: non random assignment (loss to f/u)
Recall: knowledge of presence of disorder alters recall by subjects; common in RETROSPECTIVES.
Sampling: subjects not generalizable to population
Late-look bias: info gathered at an inappropriate time (using a survey to study fatal dz, but only those alive will be able to answer the survey)
Procedure bias: subjects not treated the same; more attention given to tx group, so better adherence)
Confounding bias: occurs when factor is related to both exposure and outcome, distorting the effect of exposure
Lead-time bias: early detection confused with increased survival, seen with improved screening, but natural hx of dx is not changed, but early detection makes it seem AS THOUGH survival has increased
Observer-expectancy effect: researcher’s belief in the efficacy of a tx changes the outcome of the tx
Hawthorne effect: the group being studying changes its behavior due to the knowledge of being studied. Dr. Hawthorne is watching you.
Normal distribution
Gaussian, aka bell-shaped
Mean=median=mode
Mode is least affected by outliers
Positive skew: means>median>mode, longer tail on right
Negative skew: mean<mode, longer tail on left
SD and SEM
N= sample size, SD
SEM= SD/square rt(n)
SEM decreases as n increases
Statistical errors types
Type 1 error (alpha): mistakenly stating the existence of difference. (false positive) alpha is the probability of making a type I error.
If P
Power (1 - beta): the likelihood of finding a difference if one in fact exists. It increases with HIGH sample size, HIGH expected effect sized, HIGH precision of measurement.
Meta analysis
Pools data and integrates results from several similar studies to reach an overall conclusion, increases statistical power.
Limitation: quality of individual studies or bias in study selection.
Confidence interval
Range of values in which a specified prob of the means of repeated samples would be expecteed to fall
Cl= range from (mean+/-Z(SEM))
For 95% CI, Z =1.96
For 99% CI, Z=2.58
Null hypothesis not rejected if:
- 95% CI for a mean difference between two variables include 0.
- 95% CI for OR or RR includes 1
If the CIs between two groups do NOT overlap, significant difference exists. If DO overlap, no difference.
T test vs. ANOVA vs. X2
T test: diff btw the MEANS of 2 groups = Mr T is MEAN
ANOVA: diff btw the MEANS of 3 groups or more
ANalysis Of VAriance of 3 or more group
Chi Square: test checks difference between 2 or more % or proportions of categorical outcomes (not mean values)
Pearson’s correlation coefficient (r)
r is always between -1 and +1, the closer the absolute value of r is to 1, the stronger the linear correlation between the two variables.
Coefficient of determination = R^2, value that is usually reported.
Dz prevention
1’ prevent disease occurrence, HPV vaccination
2’ early detection, PAP smear
3’ reduce disability from dz, chemo
PDR: prevent, detect, reduce disability
Medicare and Medicaid
MedicarE is for elderly, >= 65 yrs, =<65 with cercetain disability, or those with ESRD
MedicaiD for Destitute
Both are federal programs, originated from amendments to the social security act.
MedicaiD is joint federal AND state health assistance for people with very low income.
Core ethical principles
1) Pt autonomy: obligation to respect pts as individuals and to honor their preferences in medical care
2) Beneficence: physicians have a special ethical duty to act in the pts’ best interest. May conflict with autonomy. If the pt CAN make an informed decision, ultimately, the pt has the right.
3) Nonmaleficence: do no harm. However, if the benefit of an intervention outweigh the risks, a pt may make an informed decision to proceed (most surgeries and medications fall into this categories).
4) Justice: to treat persons fairly.
