Epidemiology and Biostats Pt. 5

Question 1

Parallel vs. Serial testing

Answer 1

Parallel: 2 or more different tests run on a patient or herd at the same time, and only ONE of the test results must be positive for a positive diagnosis, or only animals that have negative results on all tests are considered free of disease

Serial: 3 or more different tests run on a patient or herd, but all test results must be positive for a positive diagnosis to be made

Question 2

Pros/cons of parallel testing

Answer 2

Pros:
-useful when you have relatively insensitive tests and want to boost the net effect of a more sensitive diagnostic strategy by running multiple tests at once
-best used on individual animals

Cons:
-number of tests increases
-risk false positive increases
-not best for groups of animals

Question 3

Pros/cons of serial testing

Answer 3

Pros:
-maximizes specificity and PPV
-more confident in positive test results

Cons:
-lowers sensitivity and NPV
-increases risk that disease will be missed

Question 4

Likelihood ratio

Answer 4

the likelihood that a given finding on the hx, PE, or lab exam would occur in an animal with the condition of interest

likelihood ratio for a positive test = sensitivity/(1-specificity)

likelihood ratio for a negative test = (1-sensitivity)/specificity

Results:
<1: result is associated with the absence of disease
>1: result is associated with disease
closer to 0: negative test
closer to 1: test conveys no information
closer to infinity: positive test

Question 5

ROC (Receiver Operating Characteristic) Curve

Answer 5

compares the true positive rate (sensitivity) on the vertical axis to the false positive rate (1-specificity) on the horizontal axis

-can be used to select cutoffs or to compare diagnostic tests.
-tests that discriminate well approach the upper left corner of the ROC curve
-the area under the curve measures overall test performance

Question 6

3 basic types of questions

Answer 6

1) close-ended questions (easier to analyze)
2) open-ended questions (harder to analyze)
3) precoded, open-ended questions (participants can answer unprompted but the interviewer selects from precoded response categories)

Question 7

active vs. passive surveillance

Answer 7

active: project staff are recruited to carry out a surveillance program

passive: available data on reportable diseases is used; used for routine notifiable diseases. Limited by variability and incompleteness in reporting

Question 8

Surveillance systems at the state and local levels

Answer 8

-communicable disease surveillance reported by health care providers
-dz registers surveillance (ie. cancer)
-sentinel surveillance (ie. influenza)
-periodic population surveys
-syndromic surveillance
-vital registration (ie. infant mortality)
-administrative data on health

Question 9

steps in planning a surveillance system

Answer 9

1. establish objectives
2. develop case definitions
3. determine data source or data collection mechanism
4. develop data collection instruments
5. field-test methods
6. develop and test analytic approach
7. develop dissemination mechanism, so that decision makers can easily understand the data and implications
8. ensure use of analysis and interpretation

Question 10

The CDC describes 7 steps in a foodborne outbreak investigation

Answer 10

1) detect a possible outbreak (surveillance, PulseNet, clinicians)
2) develop case definition and find cases
3) generate hypotheses about likely sources
4) test the hypothesis (analytic epi studies, food testing)
5) find the point of contamination (traceback, interview food prep workers, etc.)
6) control the outbreak (ie. clean/disinfect food facilities, temporarily close, recall, education, etc.)
7) decide when the outbreak is over (when number of new cases drops back to normal)

Question 11

on an epi curve, a sudden rise in the number of cases suggests:

Answer 11

a sudden exposure to a common source once incubation period earlier

Question 12

in a point-source epidemic, all cases occur within on incubation period

Answer 12

T

Question 13

How to identify the period of exposure from an epidemic curve of an apparent point source epidemic

Answer 13

1) look up the average and minimum incubation periods of the disease
2) identify the peak of the outbreak or the median case and count back on the x-axis one average incubation period
3) start at the earliest case of the epidemic and count back the minimum incubation period.

Ideally, these two dates will be similar and represent the probable period of exposure

Question 14

possible sequela to STEC

Answer 14

hemolytic uremic syndrome (HUS)

Question 15

molecular/genetic epidemiology

Answer 15

The study of distribution and determinants of disease using molecular biology
techniques. These techniques can be used in surveillance, outbreak
investigation, investigating modes of transmission, and identifying risk factors.

Question 16

The clustering at a particular value is known as the central location or central tendency of a frequency distribution.

Answer 16

T

Question 17

Two measures of spread commonly used in epidemiology

Answer 17

-range
-standard deviation

Question 18

a normal distribution is characterized by:

Answer 18

1) it is symmetrical about the mean (μ), 2) the mean, median and mode are all equal, 3) the area under the curve is symmetrical, right to left, 4) the area under the curve for a distance of +/- one standard deviation from the mean is about 68% of the total area; +/- 2 standard deviations about 95% of the total area

Question 19

binomial distribution

Answer 19

used as a model for outcomes that can be only one of two possibilities: yes/no, success/failure, sick/well, etc. The probability of “success” (p) remains constant from trial to trial. The trials are independent, where the outcome of any particular trial is not affected by the outcome of any other trial.

The 2 distribution parameters are the number of independent trials (n) and the probability of success (p). The Binomial Table is used to find the cumulative probability that a value (x) is less than or equal to some specified value

Question 20

Poisson distribution

Answer 20

contains a single parameter lamda (λ) which is the average number of occurrences of a random event in an interval of time or region of space.

The probability of observing k events in an interval of a defined length given a known rate of occurrence (λ) can be determined using the formula p(k) = where e = the natural number (2.7182818…).

Question 21

the center of gravity of a frequency distribution is the

Answer 21

mean

Question 22

geometric mean

Answer 22

the mean or average of a set of data measured on a logarithmic scale.
-used when the logarithms of the observations are distributed normally (symmetrically) rather than the observations themselves
-is always smaller than the arithmetic mean
-less sensitive to extreme values

Question 23

how to find the median

Answer 23

-middle position of all the values

middle position = (n+1)/2

if falls between 2 observations, is the average of the 2 values

Question 24

One standard deviation includes 68% of the values in a sample population and two standard deviations include 95% of the values.

Answer 24

T

Question 25

interquartile range

Answer 25

refers to the upper and lower boundary defining the middle 50 percent of observations. The upper boundary is the 75th percentile and the lower boundary is the 25th percentile.