Biostatistics

Question 1

What will you look at if you’re wondering the frequency of a disease in a population? 3 things

Answer 1

prevalence
incidence
attach rate

Question 2

What should you look at when asking how well a test differentiates sick from healthy?

Answer 2

sensitivity

specificity

Question 3

What measure will answer this question: of those in a population who test as sick or healthy, how many are truly sick or healthy?

Answer 3

predictive values

Question 4

What concepts should you look at when asking what is the impact of a treatment?

Answer 4

risk reduction

number to treat or harm

Question 5

What are the three general types of prevalence?

Answer 5

point prevalence (at one time)
period prevalence (during a period of time)
lifetime prevalence (over a course of a lifetime)

Question 6

What is the equation for prevalence?

Answer 6

number of people with a disease divided by number of people at risk for the illness at that point in time.

can change depending on how you define the at risk

Question 7

What is indicence?

Answer 7

the number of NEW cases of a disease during a time period over the number of people at risk for the illness during that time period

Question 8

True or false: incidence excludes those that already had it.

Answer 8

true - because you’re not at risk for being a new case

Question 9

What are two subtypes of incidence?

Answer 9

cumulative incidence - total number reported over time

attack rate - refers to outbreaks

Question 10

What is attack rate?

Answer 10

It’s a type of incidence used when the nature of the disease is acute and the population is observed for a short period of time

so outbreaks

Question 11

What is the equation for attack rate

Answer 11

number of new cases / number of exposed

exposure is what’s really important

Question 12

WHat is secondary attack rate?

Answer 12

It’s the number of new cases over the number of exposed minus the primary cases in the primary attack rate

so it’s a measure of disease frequency among contacts of those affected by the first exposure

Question 13

What will duration of illness do to prevalence?

Answer 13

longer illness, higher prevalence

Question 14

What will migration do to prevalence?

Answer 14

if you have lots of sick people moving into the area, the prevalence will increase

if healthy people move out of an area, prevalence will increase

Question 15

What will prevention do to prevalence or incidence?

Answer 15

should decrease incidence

Question 16

What cause the huge jump in AIDs incidence in 1993?

Answer 16

a change in diagnostic criteria

Question 17

What is the equation relationship between prevalence and incidence?

Answer 17

prevalence = incidence x (average) duration

Question 18

When will prevalence roughly equal to the incidence?

Answer 18

when the illness is super acute like the flu

Question 19

What is sensitivity?

Answer 19

the probability that a diseased person will be identified correctly by a diagnostic screening test

Question 20

What are other terms for sensitivity?

Answer 20

true positive probability or true positive rate

Question 21

What’s the equation for sensitivity?

Answer 21

true positives/total # of ill people (true positives and false negatives)

Question 22

What is specificity?

Answer 22

the probability that a well person will be identified correctly by a diagnostic screening test

aka true-negative probability

Question 23

What’s the equation for specificity?

Answer 23

true negatives / total # of well people (true negatives and false positives)

Question 24

What does it mean to have a highly sensitive, but not highly specific test?

Answer 24

It helps you catch all cases if you really want to be cautious and catch something early - if letting something go would be fatal

errs on the side of over-diagnosing

good screening test

Question 25

What does it mean for a test to have high specificity but low sensitivity?

Answer 25

Good for making sure that the person really does have the disease - good if the treatment has lots of risks

errs on the side of under-diagnosing

good for confirmation test

Question 26

What is the predictive value?

Answer 26

probability that a test will give a correct diagnosis

Question 27

WHat are the two types of predictive value?

Answer 27

positive predictive value

negative predictive value

Question 28

What are the three thigns that a predictive value will depend on?

Answer 28

1. test sensitivity
2. test specificity
3. prevalence of the disease in the population being tested

Question 29

What is positive predictive value?

Answer 29

probability that a person who tests positive actually has the disease

true positives/all positives (true and false positives)

Question 30

What is negative predictive value?

Answer 30

probability that a person who tests negative actually is healthy

true negatives/all negatives (true and false negatives)

Question 31

How does higher disease prevalence affect predictive values?

Answer 31

it makes the positive test more likely to be actually positive = increases positive predictive value

decreases negative predictive value

Question 32

How does lower disease prevalence affect predictive values?

Answer 32

it makes the negatives more likely to actually be negative

increases negative predictive value
decreases positive predictive value

Question 33

In a randomized control trial, what is the control event rate

Answer 33

proportion of the control group who have a bad outcome after “treatment”

If 10 of 30 control group participants become sicker,
CER = 10/30 = 0.33 = 33% had adverse outcomes

Question 34

What is the experimental event rate

Answer 34

proportion of treatment group participants who have a bad outcome after treatment

For example…
If 4 of 30 treatment group participants become sicker,
EER = 4/30 = 0.13 = 13% had adverse outcomes

Question 35

What is absolute risk?

Answer 35

the probability of developing a disease or undesired outcome

Question 36

How do you determine absolute risk reduction?

Answer 36

when the control event rate is higher than experimental evenr ate

CER - EER > 0

Question 37

How do you determine absolute risk increase?

Answer 37

when the control event rate is lower than experimental event rate

CER - EER < 0

Question 38

How do you determine the number needed to treat? (# of patients treated in order to save one person)

Answer 38

inverse of absolute risk reduction

NNT = 1/ARR

Question 39

How do you determine number needed to harm? (# of people affected in order to harm 1 person)

Answer 39

inverse of absolute risk increase

NNH = 1/ARI