5. Screening and sampling

Question 1

Why is screening important?

Answer 1

one health and public health are focused on prevention

Question 2

screening tests desirable qualities

Answer 2

condition screened needs to be important

screening test should have a high benefit cost ratio

condition needs to be sufficiently prevalent

an effective treatment should be avalible

screening test should have precision and accuracy

Question 3

precision

Answer 3

ability of a measuring instrument to give consistent results on repeated trials

reliability

all shots in the same space not on bulls eye

Question 4

accuracy

Answer 4

the ability of the measuring instrument to give a true interpretation of reality

validity

All shots hit bulls eye

Question 5

precision and accuracy

Answer 5

its not possible to be imprecise and accurate

is possible for a measure to be precise and inaccurate

Question 6

how to measure accuracy

Answer 6

sensitivity = ability to detect disease

specificity = ability to detect the absence of disease

positive predictive value = likelihood of disease giving positive result amount of true positive among positive results

negative predictive value = no disease giving negative results true negatives among all negative results

Question 7

characteristic of good screening test

Answer 7

high in sensitivity high in specificity high in positive predicted value and high in negative predictive value

unealistic

Question 8

realistic characteristics of screening test

Answer 8

tend to be very sensitive so you can be confident in negative result

Question 9

realistic characteristic of confirmatory test

Answer 9

tend to be very specific minimizes false positive

can be confident in positive results

Question 10

what is the gold standard?

Answer 10

the truth which we compare our tests to

definitive diagnostic test that considered best available

Question 11

chart for gold standard

Question 12

sensitivity equation

Answer 12

probability of the test being positive when the disease is actually present

true positives/ true positives + false negatives

Question 13

specificity equation

Answer 13

the probability of a test correctly classifying an individual as disease free

true negatives / true negatives + false positives

Question 14

positive predictive value equation

Answer 14

probability of a patient actually having a disease when the test is positive

true positives / true positives + false positives

Question 15

negative predictive value

Answer 15

probability of a patient being healthy when the test is negative

true negatives / true negatives + false negatives

Question 16

diagnoses are made based on what characteristics

Answer 16

clinical signs, history

regional prevalence of disease

presence of vectors

test attributes

Question 17

high PPV

Answer 17

common disease –> positive result –> probability true positive = high PPV

Question 18

low PPV

Answer 18

rare disease –> positive results –> probably false positive = Low PPV

Question 19

why sample?

Answer 19

compared to using entire population at risk sampling

costs less
less field time
acceptable accuracy
when its impossible to study entire population

Question 20

two sampling schemes

Answer 20

random sampling
non randome sampling

Question 21

random sampling

Answer 21

every element from the entire population has an equal probability of being selected

Question 22

non random population

Answer 22

probability of being chosen is unknown
less expensive
makes statistical analysis and error id more defficult

Question 23

types of random sampling

Answer 23

simple random sampling
systemic random sampling
cluster sampling
multi stage sampling

Question 24

types of non random sampling

Answer 24

convenience sample = sample is selected because they are easily accessible

snowball = friend

purposive sampling = you choose who you think

quota sampling

Question 25

components of total deviation

Question 26

sampling bias

Answer 26

sampled population is not representative of overall population

failure to allow for statistical weighing of samples of unequal popbability

Question 27

sampling error

Answer 27

difference between survey result and population value due to random nature of sample

influenced by size and scheme of sample

Question 28

measures of sample error

Answer 28

confidence limits

standard error

coefficient of variance

p values

others

Question 29

sampling error used to measure

Answer 29

sample size prior to sample
determine how sure you are of analytic results

Question 30

total deviation

Answer 30

total deviation = bias +error

Question 31

bias vs sampling error

Answer 31

bias = is due to mistake than cane be avoided cannot be precisely meausred

sampling error = unavoidable when sampling can be calculated after sample