Epidemiology Week 3

Question 1

Describe the nominal measurement scale

Answer 1

◾data that can be named and put into categories

◾categories are mutually exclusive and unordered

Question 2

Describe the ordinal measurement scale

Answer 2

◾named data

◾categories are mutually exclusive and ordered

Question 3

Describe the count/discrete measurement scale

Answer 3

◾numeric/measured data

◾specific values, integers

Question 4

Describe the continuous measurement scale

Answer 4

◾numeric/measured data

◾data that can take on any value

Question 5

What is central tendency?

Answer 5

measures of location (ex. mean, median)

Question 6

Give four measures of central tendency

Answer 6

◦Mean - arithmetic average
◦Median - middle value of sorted data, less influenced by outliers
◦Mode - most frequent value
◦Geometric mean - nth root of the product of the data

Question 7

What is variability?

Answer 7

measures of spread (ex. range, std dev)

Question 8

Give different measures of variability

Answer 8

◦Range - max value - min value
◦Interquartile range - 75th percentile - 25th percentile
◾Captures central 50% of the data
◦Variance - average squared deviation from the mean
◦Standard deviation - square root of variance
◦Coefficient of variation - standard deviation/mean

Question 9

What are the major sources of variation?

Answer 9

◦Biologic variation
   ◾Between-individual
   ◾Within-individual
◦Measurement variation
   ◾Between-observer
   ◾Within-observer
◦Instrument or analytical error

Question 10

What is reliability?

Answer 10

• Precision

- Random error

Question 11

What is validity?

Answer 11

• Accuracy

- Systematic error

Question 12

How is data displayed graphically?

Answer 12

• Histograms
• Box charts
• Bar charts/Pie charts
• Distribution curves

Question 13

Give the properties of normal distribution curves

Answer 13

◾Bell shaped
◾Total area under curve = 1
◾Extends to infinity in both directions
◾Probability corresponds to area under the curve
•68% within +/- 1 standard dev of mean
•95% within +/- 2 standard devs of mean
•99% within +/- 3 standard devs of mean
◾Only need to know mean and standard dev

Question 14

What is sensitivity:

Answer 14

Proportion of those with disease who also have a positive test result
Sensitive test means TP/(TP + FN) close to 100%
few false negatives
most negative results are true

SNOUT (Sensitive test with negative result rules out disease)
Sensitivity and negativity both have Ns in them

Question 15

Specificity

Answer 15

Proportion of those without the disease who give a negative test result
Specific test means TN/(TN+FP) close to 100%
gives few false positives
most positive results are true
Patients with positive results likely have the disease
SPIN = Specific test with Positive results rules in disease
Specificity and positive both have Ps in them

Question 16

False positive and false negative

Answer 16

If FN is worse than FP use a sensitive test

If FP worse than FN, use a specific test

Question 17

Positive predictive value

Answer 17

Calculate the probability of the patient having the disease given a positive test result
TP/(TP+FP)
prevalence is positively associated with +PV (prevalence increases, +PV increases)

Question 18

Negative predictive value

Answer 18

Calculate the probability of the patient not having the disease given a negative test result
TN/(FN+TN)
prevalence is negatively associated with -PV (prevalence increases, -PV decreases)

Question 19

Use properties of the normal (Gaussian) distribution and the standard normal (Z) distribution to estimate probabilities of clinical events:

Answer 19

normal (Gassian) distribution: describes frequency distribution of repeated measurements of the same physical object by the same instrument
standard normal distribution:
- curve is symmetrical and bell-shaped
- two-thirds of observations fall within 1 standard deviation of mean
- 95% of observations fall within 1 standard deviation of the mean

Question 20

Methods to determine reference intervals: normal distribution method

Answer 20

mean +/- 2x standard deviation
68% within +/1 1 standard dev of the mean
95% within +/- 2 standard dev of the mean
99% within +/- 3 standard dev of the mean

Question 21

Transformation method:

Answer 21

transform values to make data more like a normal distribution using logs
use the normal distribution method

Question 22

Percentile method:

Answer 22

assumes no distributional form
works for any distribution (nonparametric)
sample data should be representative of reference population
sample size should be large enough so estimates of range will be reliable