Epidemiology (IX) Miscellaneous Flashcards
Increasing the cutoff will do what to specificity & sensitivity?
Increasing the cutoff decreases sensitivity but increases specificity (fewer false positives, but more false negatives)
Decreasing it would do the opposite.
PPV increases if ___ or ___ increases
prevalence or specificity
Calculating net sensitivity in sequential testing?
of people who tested positive on test2/# of people who truly had the disease
Calculating net sensitivity in parallel testing?
# of people who tested positive on BOTH tests + ppl who tested positive on test1 + ppl who tested positive on test2 / # of people who truly have disease
Calculating net specificity in sequential testing?
# of people who tested negative on first test + # of people who tested negative on second test / # of people who actually don't have disease
Calculating net specificity in parallel testing?
# of people who tested negative on BOTH tests / # of people who actually don't have disease
In calculating the kappa statistic,
How do you find “observed agreement (%)”?
It’s overall percent agreement (a+d)/(a+b+c+d)
In calculating the kappa statistic,
How do you find “Agreement expected from chance alone (%)”?
Apply Observer A’s percentages for positive and negative to Observer B’s totals for positive and negatives.
Now, add these results and divide by the total # of cases.
Kappa can have no agreement better than chance alone, poor agreement, intermediate agreement, or excellent agreement. What is the range for intermediate agreement?
0.4 - 0.75
Epidemic Curve
# of disease cases on Y-axis Time on the X-axis
Investigation of an outbreak
define the epidemic examine the distribution of cases look for combinations of relevant variables develop hypotheses test hypotheses recommend control measures
Annual incidence rate
of new cases in a specified population during a given year
/
Midyear population estimate
Mortality rate/ Annual Crude Mortality
of deaths in a specified population during a specified time period
/
Person-time contributed by the population during the time period OR midyear population
*Either way, this is a RATE- never expressed as a percentage
Indirect adjustment tells us how risk compares between one population and a standard population. How do you calculate it?
SMR/SIR =
Observed # of deaths or disease per year
/
Expected # of deaths or diseases per year
where expected # is determined by multiplying a known population’s rates by the study population size
What’s the difference between mortality rate from disease X and case fatality from disease X?
Both have the numerator as the people who died from disease X, but
Mortality rate has the entire population in the denominator.
Case fatality has only those who were diagnosed with disease X in the denominator.
Increasing proportionate mortality doesn’t necessarily mean the disease-specific risk has increased because
People may simply be dying of other causes, which makes the denominator (total all-cause deaths in population during time period) larger.
The first step in forming a cohort study’s study population is
to exclude prevalent cases from the reference population!
Confounding
Exposure is associated with other factors that might influence the outcome, thus distorting the association between exposure and risk
Information bias
AKA measurement error.
Systematic mis-measurement of exposure or disease, especially if the quality or extent of information obtained is different between groups.
Selection bias
Including nonparticipants or people lost to follow-up
Total population vs defined population vs study population
Total population: overall population group
Defined/source/reference population: Population that the trial is meant to be applied to
Study population: people actually in the study
Primary goals of randomization in cohort studies
- Removing investigators’ biases
- Increase study groups’ comparability in target and non-target characteristics (can also be done by having a larger sample size), though not always guaranteed
How can you guarantee comparability in cohort study arms/groups?
Stratify before randomizing!
Ex) Distribute half the men and half the women to the control group and the other halves to the experimental group so that both groups have equal amts of men and women.
Data and safety monitoring boards must:
- Ensure that any risks to participating are minimized
- Ensure integrity of data
- Stopping trials for safety reasons; if objectives won’t be met; or if it becomes clear objectives will be met and it’s unethical to continue