Epidemiology & Biostats Flashcards
The specific morbidity rate is usually the number of:
a. Cases of a specific disease per 1,000,000 population
b. Deaths from a specific disease for a geographical area
c. Cases of a specific disease for a political area
d. Deaths from a specific disease per 100,000 population
e. Deaths from a specific disease per 100 cases of that disease
A. Morbidity relates to who gets sick from an illness. The denominator can be persons if the time period is specified, or person-years.
For the following 2x2 table for a diagnostic test, write out expressions for each term:
Disease + Disease -
Exposure + A B
Exposure - C D
a. Specificity
b. PPV
c. Total population
d. NPV
e. Prevalence
f. Sensitivity
a. Specificity: d/(b+d)
b. PPV: a/(a+b)
c. Total population: a+b+c+d
d. NPV: d/(c+d)
e. Prevalence: (a+c)/(a+b+c+d)
f. Sensitivity: a/(a+c)
The amount of health disorder existing in a population at one particular time, regardless of time of onset is known at the:
a. Prevalence
b. Incidence
c. Morbidity rate
d. Mortality rate
e. Attack rate
a. Prevalence
In prospective study or cohort type of epidemiologic study, two types of cohorts are selected. One of these is the exposed and the other is______; the measure of effect used in this study is _________
a. Cases; odds ratio
b. Susceptible; risk ratio
c. Affected ; relative risk
d. Non-exposed; incident rate ratio
e. Immune populations; odds ratio
D. Non-exposed; incident rate ratio
Textbook definition of a cohort study – following exposed and non-exposed over time to see who develops the disease.
The duration of a chronic disease process may complicate the epidemiologic study of its prevalence because of:
a. Loss of people or animals from the study by death from other causes
b. Changes in diagnostic techniques during the period of study
c. Changes in medical or veterinary care during the period of study
d. Decrease in interest level on the part of workers in the study
e. All of the above
E. All of the above
Pretty common sense, but in essence, if you want to know who is infected in a population, drop-out, changes in how a disease is diagnosed or treated can change your case definitions, your degree of ascertaiment of cases, etc.
When an epidemiologist is called to investigate a communicable disease emergency, the first thing he/she should try to determine is:
a. Possible sources of infection
b. Methods of transmission
c. Accuracy of the diagnosis
d. Methods of control
e. Extent of spread
C. Accuracy of the diagnosis
Making sure you know what disease you’re dealing with is the first step needed before asking any other questions
The occurrence in a community or region of cases of an illness in the human population clearly in excess of normal expectancy and derived from a common propagated source is an:
a. Epidemic
b. Endemic
c. Pandemic
d. Epizootic
e. Anthropozoonosis
A. Epidemic
This is the textbook definition of an epidemic.
Which of the following agent characteristics is most likely to be seen in a disease which occurs in epidemic proportions:
a. High infectivity
b. High pathogenicity
c. High virulence
d. Low antigenicity
e. Viability
A. High infectivity
For this question, I return to my definition of the R0, which is the average number of new cases of an infection caused by one typical infected individual, in a population consisting of susceptibles only. When R0>1, an epidemic occurs. R0 depends on the transmissibility or infectivity of the agent, the contact rate between hosts, and the time spent infectious. So if the infectivity of the agent increases, you’re more likely to have an epidemic.
In a study of alcohol and oral cancer the relative risk is 2.0 for men and 2.0 for women but 4.0 for both sexes combined. This suggests that:
a. There is confounding by sex in these data
b. There is confounding by some unknown or unmeasured factor in these data
c. There is evidence of effect modification in these data
d. The results have been adjusted for age and sex
e. The results are due to bias
A. There is confounding by sex in these data
General rule of thumb is that when you stratify by your variable of interest, it will be a confounder if both stratified effect estimates are similar and more than 10-15% different from the crude estimate.
A new treatment is developed that prevents death but does not produce recovery from disease. Which of the following will occur?
a. Prevalence will increase
b. Prevalence will decrease
c. Incidence will increase
d. Incidence will decrease
e. None of the above
A. Prevalence will increase
Think of prevalence as a water in a bucket. Prevalence increases when water (i.e., people with the disease) are added to the bucket. Prevalence decreases when there is a hole in the bucket and water is leaving the bucket (i.e., people are recoverying or dying and leaving the population). If more cases keep arriving but there are no departures from the population, the prevalence will increase.
In a country where a disease is endemic:
a) The number of affected animals tends to stay more or less constant over time
b) There have been at least 2 outbreaks of that disease in the past 5 years
c) The disease has persisted in that population for a long time
d) The vaccine for that disease is probably not used in a widespread manner
C. The disease has persisted in that population for a long time
The definition of an endemic is one that is regularly found among particular populations or in a certain area – so nothing about numbers or vaccination.
As a dairy practitioner, you read with great interest a recent paper describing a clinical trial testing a new drug to treat mastitis. This drug, called Masticate™, is touted as costing half the price and being easier to administer than most other therapies for mastitis. The paper tested this drug against the current standard of care, and the authors found no difference in cure rates. You decide to try it in your herd. Six months later, you find that Masticate™ is actually less effective than the drug you used before – whereas before, your cure rate was around 80%, now your cure rate is closer to 65%. Which of the following is the LEAST likely possible explanation for the discrepancies between your experience and the findings reported in the paper?
a) The sample size in the original paper was small, and therefore the study was underpowered to detect the difference you found.
b) The authors of the paper defined a successful cure differently than you did.
c) The animals enrolled in the study were primiparous cows only; your herd has a mix of different aged cows, and the results may therefore not have been generalizable to your herd.
d) The authors were not blinded to the treatment and therefore could have scored the cows receiving Masticate™ more generously.
e) The batch of drugs you used was defective.
E. The batch of drugs you used was defective.
All of the other options are very reasonable explanations of why two “studies” (i.e., the published paper and your experimental trial) would have found different answers.
A case control study compared the amount of daily coffee drank by patients with pancreatic cancer (cases) and patients with other GI conditions (controls). The study found a dose-response association between drinking coffee and pancreatic cancer that persisted when adjusting for cigarette smoking. What is the most likely explanation for the findings of the study? For bonus points, provide an explanation for why.
a) A true association – drinking coffee causes pancreatic cancer (yikes!)
b) Information bias
c) Selection bias
d) Confounding
- C. Selection bias
i.e., who gets into or stays in the study. This is a historical example so you may have heard of it, but you can come up with a likely explanation. Because controls often had GI issues such as esophagitis, ulcers, etc., they self-limited coffee consumption. Their coffee consumption was lower than that of the general population, so it appeared that cases drank more coffee. The controls were not representative of the general population to which we would like to extrapolate our findings, so we have an issue of selection bias here. D. Confounding is also a possibility – we controlled for smoking but there could be other unmeasured confounders.
In a country with a population of 6 million people, 60,000 deaths occurred during the previous year. These included 30,000 deaths from cholera in 100,000 people who were sick with cholera.
What was the cause-specific mortality rate from cholera during the previous year?
a. 5%
b. 10%
c. 50%
d. 5 per 1000
e. 10 per 1000
D. 5 per 1000
Cause-specific mortality rate per 1,000 population = # of deaths from that cause/# of people in the population x 1000 = 30,000 cholera deaths/6 million population x 1000 = 5 per 1000.
In a country with a population of 6 million people, 60,000 deaths occurred during the previous year. These included 30,000 deaths from cholera in 100,000 people who were sick with cholera.
What was the case-fatality from cholera in the previous year?
a. 1%
b. 5%
c. 10%
d. 30%
e. 50%
D. 30%. Case fatality rate = # dead from the disease/# with the disease
Which one of the following frustrations would you most likely expect in preparing to carry out cohort studies on animal disease?
a. Costly and time consuming, and plagued by the continual changing of the cohort.
b. Difficult time in selecting a comparison group or control population upon which to test your hypothesis.
c. Cohort populations are unchanging, and that no new individuals are introduced into the study population.
d. Data collected retrospectively is often incomplete, and plagued by high degrees of institutional bias.
e. Unable to get accurate estimates of incidence or prevalence of the disease using the cohort study technique.
A. Cohort studies, especially prospective cohort studies, tend to be more expensive, and theytake longer to conduct because you’re following forward in time. I would argue D is also anacceptable answer, as investigators using retrospective data have much less control over thecohort and less ability to be confident in the completeness of their data. Answers b and e tendto apply more to case-control studies.
One of your clients has a feedlot containing 15,000 cattle, 10,000 of which are susceptible. In a current outbreak of disease, 3,000 became sick and 300 died. The case fatality rate was:
a. 10%
b. 25%
c. 2%
d. 30%
e. 3%
A. CFR=the proportion of animals that die from a specified disease among all individuals diagnosed with the disease over a certain period of time
What study plan would be best to determine the effectiveness of a new vaccine in preventing disease in humans?
a. Case-control study.
b. Cohort study.
c. Prevalence study.
d. Morbidity study.
e. Retrospective study.
B. For an observational study, you want to compare outcomes among who was exposed, i.e.,vaccinated) and unexposed (non-vaccinated). For an experimental study, a randomized controltrial would be better!
A certain causal factor is thought to be associated with an extremely rare disease. What study plan would yield the best data with limited financial and human resources?
a. Prevalence study.
b. Case-control study.
c. Prevalence study.
d. Morbidity study.
e. Case evaluation study
B. Case control studies are better for rare diseases, because you don’t have to wait for cases of arare disease to accumulat
Match the following terms to their definitions:
A. A causal factor that is neither necessary nor sufficient, but increases the likelihood of disease, all other things being equal.
B. Any factor that must be present for the disease to occur.
C. Any factor or, more commonly a constellation of factors, that inevitably lead to the disease
i. Sufficient cause
ii. Necessary cause
iii. Contributing cause
A. contributing
B. Necessary
C. Sufficient
The measure most sensitive to extremes is:
a. Mean
b. Median
c. Mode
d. Sample
e. Inferential
A. The mean is most susceptible to outliers. That is why when we have non-normally distributedor skewed data, it is more appropriate to present the median when performing descriptivestatistics.
Generalizability is best assured by:
a. Representative nature
b. Randomness
c. Sample size
d. Precise manipulation
e. Statistical validity
A. Generalizability indicates how well your results are likely to apply to other populations. If your sample population is representative of other populations, then it is likely your results are generalizable.
Which of the following is NOT associated with a retrospective study?
a. Adaptable to conditions of low prevalence.
b. Less expensive than prospective.
c. Requires fewer personnel.
d. Takes longer to conduct.
e. Provides less accurate incidence rate.
D. If your data are retrospective (i.e., already collected), then you take out the time factor ofhaving to follow your population over time and accumulate cases with your desired outcome
An epidemic curve displays:
a. The population at risk versus the frequency of cases.
b. The frequency of cases versus the number of ill in the population.
c. The time of onset versus the population at risk.
d. The time of onset versus the frequency of incident cases.
e. The time of onset versus the number of individuals who are ill.
D. An “epidemic curve” shows the frequency of new cases over time based on the date of onsetof disease.
A decrease in the prevalence of a disease could be interpreted as a result of:
a. A reduction in the incidence.
b. A more rapid cure.
c. A shorter life span of affected individuals.
d. “a” and “c” above.
e. All of the above.
E. Again, if you consider the analogy of water in a bucket, with more water (disease individuals)entering the bucket and some leaving through a hole in the bucket, the total amount of water inthe bucket (prevalence) can decrease if the incidence is lower (less water coming in), or if moreare leaving the bucket through the hole (either by dying or becoming recovered).
In the hierarchy of Scientific Evidence, what types of study provide the highest levels of evidence?
a) Cohort studies
b) Cross sectional studies
c) Randomized clinical trials
d) Meta-analyses and systematic reviews
The conventional wisdom is that SR/MAs provide the highest level of evidence, because you aresummarizing all of the available evidence. However, keep in mind that with a SR/MA, it is“garbage in, garbage out”, so a SR/MA is only as good as the studies that go into it
On a poultry farm, all of the birds are checked every day by their keepers for signs of disease and mortality. When farmers find a sick or dead bird, they alert veterinary services and can send the bird in for examination/autopsy and infectious disease testing. This is an example of:
a) Active surveillance
b) Passive surveillance
c) Targeted passive surveillance
d) Sentinel surveillance
Passive surveillance is when animals/people come to our attention becausethey are suspected of being cases – i.e.,a system by which a health jurisdictionreceives reports submitted from hospitals, clinics, public health units, or other sources. Sincethe lab is waiting for these dead birds to be sent to them by the farmer, this is passivesurveillance.
An investigator is performing a study to examine the effect of being overweight on experiencing CCL tears in dogs. The investigator also wants to control for age and spay/neuter status. What statistical technique does the investigator need to use to test her hypothesis?
a. Chi-square test
b. T-test
c. Linear regression
d. Logistic regression
e. Ordinal regression
D. Logistic regression – since the outcome is categorical (injury or no injury)and multiple predictors are being examined, logistic regression is needed.Chi-square test could be used if we were only looking at effect of beingoverweight on CCL tears, but since we want to adjust for other factors, weneed a multivariable model.
Two veterinarians want to investigate a new laboratory test that identifies streptococcal infections. Dr. Kidd uses the standard culture test, which has a sensitivity of 90% and a specificity of 96%. Dr. Lamb uses the new test, which is 96% sensitive and 96% specific. If 200 animals undergo culture with both tests, which of the following is correct?
a. Dr. Kidd will correctly identify more animals with streptococcal infection than Dr. Lamb
b. Dr. Kidd will correctly identify fewer animals with streptococcal infection than Dr. Lamb
c. Dr. Kidd will correctly identify more animals without streptococcal infection than Dr. Lamb
d. The prevalence of streptococcal infection is needed to determine which vet will correctly identify the larger number of animals with the disease
B. Sensitivity is how well a test can detect a case of the disease. Lower sensitivity means thatthe test will pick up, or correctly identify, fewer cases of the disease
In a colon cancer screening study, individuals 50 to 75 years old are screened with the Hemoccult test. In this test, a stool sample is tested for the presence of blood. If the Hemoccult test result is negative, no further testing is done. If the Hemoccult test result is positive, the individual will have a second stool sample tested with the Hemoccult II test. If this second sample also tests positive for blood, the individual will be referred for more extensive evaluation. What is this type of screening called, and what is the effect on net sensitivity and net specificity of this method of screening?
a. Serial testing; Net sensitivity and net specificity are both increased
b. Serial testing; Net sensitivity is decreased and net specificity is increased
c. Parallel testing; Net sensitivity remains the same and net specificity is increased
d. Parallel testing; Net sensitivity is increased and net specificity is decreased
e. Parallel testing; Net sensitivity is decreased and net specificity is increased
B. This is serial testing, which reduces sensitivity but increases specificity. This is because, in series testing, it is harder to be a “true” negative (you need to have two tests be positive to say you have the disease), but it is easier to be a “true” negative (only need one negative result tobe called disease-free)
A diagnostic test has been introduced that will detect a certain disease 1 year earlier than it is usually detected. Which of the following is most likely to happen to the disease within the 10 years after the test is introduced? (Assume that early detection has no effect on the natural history of the disease. Also assume that no changes in death certification practices occur during the 10 years.)
a. The period prevalence rate will decrease
b. The apparent 5-year survival will increase
c. The age-adjusted mortality rate will decrease
d. The age-adjusted mortality rate will increase
e. The incidence rate will decrease
B. This should be fairly intuitive. If the detecting the disease earlier does not change the course of the disease, then it will just look like people are “surviving” longer.
Investigators enrolled 100 diabetics without eye disease in a cohort study. The results of the first three years are as follows:
Year 1: 0 cases of eye disease detected: 8 lost to follow-up
Year 2: 2 new cases of eye disease detected; 2 patients died; 10 lost to follow-up
Year 3: 3 new cases of eye disease detected; 2 more patients died; 13 more lost to follow-up.
The person-time incidence rate is calculated as:
a. 5/100
b. 5/63
c. 5/235
d. 5/205
D. The person-time incidence rate is the number of new cases divided by the total person-time at risk. What’s important to remember is that it is often assumed that someone who didn’t make it the full year (i.e., was lost to follow-up or was diagnosed or died during the year is assumed to have contributed half of the observed time).Ideally we would have much more precise records where we would know exactlywhen a person became ill or dropped out, but when we don’t we make thismiddle-ground assumption. So here’s the breakdown for this particular study
Define the terms “sensitivity” and “specificity” as they relate to diagnostic test interpretation.
Sensitivity and Specificity are conditional probabilities that describe the performance of a diagnostic test. Sensitivity is the probability that a positive test result will be obtained, given the condition that the individual tested has the disease. Specificity is the probability that a negative test result will be obtained, given the condition that the individual tested does not have the disease
Explain why it would, or would not, be appropriate to estimate “relative risk” from atypical cohort study.
Relative risk (i.e., risk ratio) is the appropriate measure of association used in cohort studies. Relative risk is calculated as: (risk of disease in exposed group)/(risk of disease in unexposed group). Risk is defined as the number of new cases that occur during a specific time period in a specific population (e.g., exposed or unexposed).Cohort studies follow exposed and unexposed individuals over time to determine who develops the outcome of interest. Because of this, incidence (and risk) of the
Week 1 Practice SME Answer Key2013 ACVPM Epi. Exam
outcome of interest can be estimated, making relative risk an appropriate measure of association for cohort studies
In general, what does it mean when there is a significant interaction between two predictor variables in a multivariable regression model?
In a multivariable regression model, a statistical interaction exists betweentwo explanatory (i.e., predictor or independent) variables when the effect ofthe each variable on the outcome depends on the effect of the other variable.In other words, the effect of each variable on the outcome is not independentof the other variable. The combined effect of the two variables differs from the sum of the individual effects of each variable.
n a case-control study to investigate atresia coli in cattle, it was found that Holstein-Friesian calves were significantly more likely to have atresia coli than all other breeds combined.
a) What statistical test was used to make this determination?
b) What is the null hypothesis for this particular test (you can use mathematical notation or English)?
a) Chi-square
b) No association between breed of cattle and atresia coli
Women who had been exposed to a pesticide, DDE, were followed for 20 years. At the start of the study period, the women completed a questionnaire and had blood drawn, and the women were classified as having either low dose or high dose exposure. Of the 792 women who had high dose exposure to DDE, 430 were later diagnosed with breast cancer. Of the 3,525 women with low dose exposure to DDE,1,079 were later diagnosed with breast cancer.
a) What type of study design is this?
b) What is the cumulative incidence of breast cancer for those exposed to high doses of DDE?
c) What measure of association is appropriate to calculate for this study design?
d) Please calculate that measure of association. Please show all calculations, including the 2-by-2 table.
e) Interpret the calculated measure of association.
f) Please calculate the attributable risk percent.
g) Interpret the attributable risk percent.
a) Cohort study
b) 430 / 792 = 54%
c) Relative risk (RR)
d)
Present Absent Total Risk
High 430 362 792 430/792 = 0.54
Low 1079 2446 3525 1079/3525 = 0.31
RR = 0.54/0.31 = 1.74
e) Women who had high dose exposure to DDE had 1.74 times the risk of developing breast cancer than did women who had low dose exposure to DDE.
f) You will remember that the attributable risk percent (attributable fraction among the exposed) is calculated by Risk exposed– Risk unexposed) / Risk exposed which can be reduced to (RR – 1) / RR or (OR – 1) / OR(1.74 – 1) / 1.74 = 43.5%g) Interpret the attributable risk percent. If high dose exposure to DDE were prevented in the group of women exposed to high dose DDE exposure, we would prevent at most 43.5% of the breast cancer cases in that group.
g) If high dose exposure to DDE were prevented in the group of women exposed to high dose DDE exposure, we would prevent at most 43.5% of the breast cancer cases in that group.
What are three ways to control for confounding in epidemiologic studies?
Randomization
Restriction
Matching
Stratification
Multivariate analysis
The following question is on a questionnaire designed to investigate the effect of coffee consumption on cardiovascular disease. How much coffee do you drink?
i. 1 cup
ii. 2-3 cups
iii. 3-5 cups
iv. More than 5 cups
a) Describe two things that are wrong with this question and its available answers.
No timeframe (per day, per week, etc.)Not all possible options for answers are listed (“none” is not an option, etc.)
Categories are not mutually exclusive “Cup” not defined
What is the ecologic fallacy?
Ascribing characteristics and associations demonstrated at the group level to individuals
Fifteen hundred adult males working for Lockheed Aircraft were first examined in1951 and were classified by diagnosis criteria for coronary artery disease. Every 3 years they were examined for new cases of this disease; attack rates in different subgroups were computed annually. This is an example of a:
a) Cross-sectional study
b) Prospective cohort study
c) Retrospective cohort study
d) Ecologic study
e) Case-control study
b) Prospective cohort study
Which of the following is not an advantage of a prospective cohort study?
a) Incidence rates can be calculated
b) Precise measurement of exposure is possible
c) Recall bias is minimized compared with a case-control study
d) Many disease outcomes can be studied simultaneously
e) It usually costs less than a case-control study
e) It usually costs less than a case-control study
One hundred patients with infectious hepatitis and 100 matched neighborhood wellcontrols were questioned regarding a history of eating raw calms or oysters withinthe preceding 3 months. What kind of study design is this?
a) Cross-sectional study
b) Prospective cohort study
c) Retrospective cohort study
d) Ecologic study
e) Case-control study
e) Case-control study
All of the following are important criteria when making causal inferences except:
a) Replication of findings
b) Temporal relationship
c) Null hypothesis
d) Strength of association
e) Biologic plausibility
c) Null hypothesis
Geographic variations were determined in the incidence of inflammatory bowel
disease (IBD). Incidence of IBD was observed highest in areas with higher
socioeconomic status, the lowest rates of enteric infection, and with the highest rates
of multiple sclerosis. This is an example of a:
a) Cross-sectional study
b) Prospective cohort study
c) Retrospective cohort study
d) Ecologic study
e) Case-control study
d) Ecologic study
A case-control study is characterized by all of the following except:
a) Study participants are selected based on disease status
b) Assessment of past exposure may be biased
c) It is relatively inexpensive compared with most other epidemiologic study designs
d) Incidence rates may be computed directly
e) Definition of cases may be difficult
d) Incidence rates may be computed directly
You are evaluating a new diagnostic test by comparing it to a gold standard.
a. What is the sensitivity of the test?
b. What is the specificity of the test?
c. What is the predictive value positive of the test?
d. What is the predictive value negative of the test?
e. What is the prevalence of disease in this example (based on the results of the gold standard test)?
f. What happens to the predictive value positive if the prevalence decreases?
a. Sensitivity = TP / Total with disease = 260 / 325 = 80%
b. Specificity = TN / Total without disease = 1640 / 1735 = 95%
c. PVP = TP / Total positives = 260 / 355 = 73%
d. PVN = TN / Total negatives = 1640 / 1705 = 96%
e. 325 / (325 + 1735) = 15.8%
f. If prevalence decreases then PVP decreases
Epidemiologic models can be useful for all of the following except:
a. Predicting effectiveness of programs
b. Organizing and storing knowledge about a disease process
c. Predicting risk or consequences of disease
d. Identifying an individual’s risk factors for disease
e. Developing policy
d. Identifying an individual’s risk factors for disease
To be effective, surveillance systems should incorporate all of the followingexcept:
a. Generation of information for action
b. Disease eradication
c. Ongoing data collection
d. Systematic data collection
e. Timely information dissemination
b. Disease eradication
Surveillance system design should aim at
a. Maximizing the probability of true early detection
b. Incorporating as many sampling architectures as possible
c. Minimizing the probability of a false-positive alarm
d. All of the above
e. a and c only
e. a and c only
It is important if treatment at the pre-symptomatic stage has a more favorable outcome than treatment initiated once the patient is symptomatic.
T/F
True
The lead time is defined as the period in the natural history of the disease in which treatment is more effective and/or less difficult to administer.
T/F
False
The lead time is defined as the interval by which the time of diagnosis is advanced by early detection of disease through screening compared with the usual time of diagnosis.
The critical point in the natural history of a disease is the point before which treatment is more effective and/or less difficult to administer.
In order for a screening program to be effective, there does not need to be an accepted treatment for patients identified with the disease.
T/F
False
If α is our false-positive error rate, or the probability of making a Type I error, and β is our false negative error rate, or the probability of making a Type II error, what is power?
Power is 1 – β, the probability of detecting a difference if one truly exists
For a given α and measure of association, how can an investigator increase the power of a study?
Increase the sample size
Probability sampling
a. Refers to several sampling strategies
b. Allows investigators to generalize the results from the sample to the population
c. Allows calculation of the standard error of the resulting population estimates
d. All of the above
e. a and b only
d. All of the above
A true lack of association may be difficult or impossible to distinguish from a true association that cannot be detected statistically because of inadequate _________.
a. α (alpha)
b. β (beta)
c. Power
d. Detection rates
e. Error rates
c. Power
The use of a Geographic Information System (GIS) allows an investigator to assessall of the followingexcept:
a. The cohort effect
b. Whether there is a spatial pattern
c. Whether patterns co-distribute
d. If risk factors differ with location
e. How disease spreads
a. The cohort effect
