Epidemiology 250B Measures of Diseasae Module I Flashcards

Terms Definitions
Define rate from slides in class. Give epi example.
The changes in one quantity per unit change in another quantity. (example: incidence density)
Define ratio from slides. Give an example of one with a dimension and one without.
A fraction where the numerator is not included in the denominator. (example w/ dimension: # of hosp. beds/1,000 people; example w/o dimension: # of epi students/ non-epi students among MPH students
What is a common epidemiological ratio? Formula? (hint: only actual measure you can get from case-control data)
odds ratio
[ (# Dexp/ Not D w/exp) / (# D w/o exp / # not D w/o exp) ]
Define prevalence ( aka prevalence proportion) and give an epidemiological example.
The proportion of a population with a disease determined at a specific point in time.
Define odds in an epidemiological manner.
The # of persons in a population with a disease divided by the the # of persons in a population without a disease.
Give an example of prevalence using swine flu in the berkeley school of public health.
There are 5 cases of swine flu amongst the 200 MPH students. Thererfore PP = 5/200 = 0.025
Define incidence. Give an example. Is it a rate, ratio or proportion?
The frequency of development of new cases of disease in a population. There was 5 cases of swine flu in 200 person-years. It is a rate.
Define cumulative incidence.
The proportion of subjects at risk for a disease at baseline who develop the disease during a certain period of time. (ex. 20 nervous break downs per one week of finals at Berkeley)
Does cumulative incidence (CI) have a dimension?
No bc it is a proportion and ranges from 0 to 1.
In 18 months, out of 10,000 shoppers visiting the dollar store, 25 of them come down with cleptomania due to the economic crisis. What is the CI for the 18 mo. period?
CI = 25/10,000 = .0025 or 0.25%
Define incidence density. Give an example.
The # of incident (aka new cases) divided the total person time. (ex.20 cases of diptheria in 8,000 person years)
What is the unit for prevalence?
cases per persons at risk
What is the unit for an odds ratio?
None --it is dimensionless
What is the two different units for incidence density?
# of cases per person time OR 1/ time (according to Klienbaum)
1,000 men with no history of CVD are observed for 24 months. 10 of them develop CVD on the first day of the 13th month. What is the cumulative incidence?
CI = 10 / 10,000 = 0.001 or 0.1% per 24 months or 0.001 per 2 years.
Define crude rate.
The # of experiences or events in a total population in certain period.
Give two common examples of crude rates.
1. crude birth rate
2. crude death rate
Define adjusted rate.
A mathmatical transformation of a crude rate to account for differences in population structure (i.e. age, race, etc)
Define case fatality rate
# of cause-specific dths among cases/ # of incident cases of the same cause
Define attack rate
# of incident cases of disease/ total population at risk over a restricted period of time
What is attack rate commonly used for?
outbreak investigations
Is case fatality rate a rate, ratio, or proportion?
Although is says "rate" in its name, it is actually a true proportion
What is postnatal mortality rate?
index of the risk of death in infants aged 1- 11 months
Define postneonatal mortality rate.
# of dths in children aged 1-11 months / # of live births during that calendar year
What is infant mortality rate most commonly used for?
risk of death during the first year of life
Define infant mortality rate.
# of infant deaths in a calendar year/ # of live births during the same calendar year
Define crude birth rate. How is it expressed?
# of live births during a given time interval / # estimated total population at a given mid- interval

Expressed as per 1,000
Define curse fertility rate. How is it expressed?
# of live births during a given time interval/ estimated # of women aged 15 -44 years

Expressed per 1,000
Define maternal mortality rate.
# of deaths in a year from puerperal causes/ # of live births during the same year

**all live births, not all pregnancies
How is maternal mortality rate expressed?
per 100,000
Define the crude rate of natural increase in terms of natality rates.
(# of live births - # of deaths during a given interval) / estimated total population at a given interval
How is crude rate of natural increase expressed?
per 1,000
Define low birth rate ratio.
# of live births under 2500 gm during a given interval / # of live births during a given interval
Give the formula for prevalence using incidence of a rare disease and does not have long duration.
P = Incidence * duration of disease
What is the formula for prevalence using incidence when the disease is not rare OR the disease has a very long duration?
P = (ID* T) / (ID * T) + 1
Give the formula to estimate risk using the simple cumulative method.
R = CI = I (t0 , t) / N'0

The # of incident cases between time zero and the end of the study divided by the total number of participants at the beginning of the study (disease free of course)
What is sensoring in terms of risk estimates?
The inaccurate portrayal of risk estimates because participants were not followed the entire period or the lack of information conc???
How does one estimate the "effective number" of persons at risk using the actuarial method?
= (N'o - W/2)

The number of persons beginning the time interval minus half of the # of withdrawls.
What measure of disease is used to test "etiologic hypotheses"?
Incidence density
What are the three ways to measure person time?
1. sum of how much each individual time each person was in study
2. "steady state" pop --people come in and out often enough to assume PT = N * dt
3. Don't know indiv. PT as there is no "steady state" so just use mid-interval disease free population as the avg. pop. size
What are the three ways to calculate CI or risk?
1. simple cumulative
2. actuarial
3. density method
What assumption is made when using the simple cumulative method?
that are study participants are followed for the entire study either till the end or until they get disease --aka no death to competing causes or withdrawls
What is an assumption made when using the actuarial method to calculate risk?
Withdrawls occur mid-interval of observation time
What a major assumption made when using the density method?
a closed cohort with no competing risk and the age interval ID remain constant over that ID(not sure what this means--will ask in OH)
What is the most exact method for calculating risk?
density method
What are the three assumptions that must be met in order to use CI instead of ID in the density method?
1. rare disease (< 10%) --from rare disease assumption
2. event is inevitable --no competing risks (rate is constant w/in each age interval)
3. each age-specific ID is constant over that time period
Which of the three methods for calculating risk assumes not censoring or withdrawls?
simple cumulative
What measures of disease can be used for cross-sectional studies
1. point prevalence --only existing cases included
2. period prevalence -- longer time frame, can include both new and existing cases
What MOD is best for follow-up studies/prospective studies?
Since only new cases are measured--cumulative incidence or incidence density
ratio, rate, or proporiton:
1. CI
2. ID
3. prevalence
4. odds
5. risk ration
6. rate ratio
1. proportion
2. rate
3. proportion
4. ratio
5. ratio
6.ratio
What method uses rates to calculate risk?
Density method
Risk or rate?
1. instaneous potential for change in disease status per unit change in time
2. cumulative incidence
3. probability that an indiv. will develop outcome during a specified time pd., conditional on not dying from something else
4. incidence de
1. rate
2. risk
3. risk
4. rate
5. rate
6. risk
7. risk
8. rate
9. risk
10. rate
Can you compare SMR's between studies?
Nope
Can you compare SRR's between studies?
Yep.
What are 4 ways to reducing confounding?
1. stratificatin
2. adjustment
3. propensity scores
4. inverse probability of treatment weighting (IPTW)
What are some common adjustment techniques?
1. linear regression
2. logistic regression
3. poisson regression
4. proportional hazard models
What is simpson's paradox?
thinks hats --if that helps

it is the reversal of cause/effect when the samples are combined
List three methods in study design to reduce confounding.
1. Matching.
2. Randomization
3. Restriction
List two method in study analysis to reduce confounding.
1. Multivariate analysis
2. Stratify
List the three criteria for confounder
1. not on the causal pathway
2. a risk factor for the disease in the unexposed
3. be associated w/ exposure variable
True/False
1. If it is a confounder then it has to be an effect modifier
2. If it is an effect modifier then it has to be a confounder
1. False
2. False
What it the null hypothesis for the test for homogenity?
The pooled estimator (ORmh) is not different from the stratified estimates.
How do you calculate the degrees of freedom for the test for homogenity?
N (# of strata) - 1
What does rejecting the null hypothesis indicate?
That there may be effect modification and that it is necessary to report the stratified estimates rather than the overall effect estimator
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