Statistics

Question 1

type of study where investigators evaluate characteristics of a certain patient population without performing any interventions?

Answer 1

observational study

Question 2

List the types of studies in order of increasing strength of evidence that can be obtained

Answer 2

case report, case series, case-control, cohort, randomized controlled double-masked trial, meta-analysis

Question 3

Describes a finding in regard to a single patient

Answer 3

case report

Question 4

Investigate a group of patients without standardizing the patient information

Answer 4

case series

Question 5

What type of study: Retrospectively analyze group of patients with CNV 2/2 AMD and evaluate the benefit of laser v PDT

Answer 5

case series

Question 6

Compare a group of people with a disease (cataracts) to a group of patients without the disease, and compare the proportions of patients in each group who were exposed to a specific risk factor (smoking)

Answer 6

case-control

Question 7

Compare a group of people with a specific risk factor (diabetes) compared a group without, and follow the groups to determine relative proportions that develop a disease (heart attack)

Answer 7

cohort study

Question 8

Researchers select pateitns with and without a disease from a current database and obtain history by survey or review of medical records, thus obtaining data about exposure to risk factors right away

Answer 8

case-control

Question 9

which type of study design does recall bias have the greatest effect?

Answer 9

case-control

Question 10

correlates exposure and risk factors with the prevalence of disease without knowing the timing or sequence of exposure and disease development

Answer 10

cross-sectional study

Question 11

What type of study: collect blood from a sample of patients and compare cholesterol levels to lens status (phakic, pseudophakic, aphakic)

Answer 11

cross-sectional study

Question 12

randomly assigns patients to different treatment groups

Answer 12

clincal trial

Question 13

definition of and equation for sensitivity

Answer 13

proportion of patients with a disease who are determined to have the disease by a particular test

sens = true positives / (true pos + false neg)

Question 14

definition of and equation for specificity

Answer 14

proportion of patients without a disease who are not determined to have the disease by a test

spec = true neg / (true neg + false pos)

Question 15

definition of and equation for positive predictive value

Answer 15

proportion of patients that are determined to have a disease by a particular test who actually have the disease

PPV = true pos / (true pos + false pos)

Question 16

definition of and equation for negative predictive value

Answer 16

proportion of patients with a negative test result that actually do not have a disease

NPV = true neg / (true neg + false neg)

Question 17

low disease prevalence leads to artificially high ___ and artificially low ___

Answer 17

high false positives

low positive predictive value

Question 18

x and y axis of receiver operating curve

Answer 18

x: 1 - specificity
y: sensitivity

Question 19

what on a receiver operating curve correlates to the degree of diagnostic precision of a test?

Answer 19

area under the curve

Question 20

what is the likelihood ratio for a positive test? for a negative test? which is more important if you are trying to diagnose a disease, and which is more important to rule out a disease?

Answer 20

positive: sensitivity / (1 -specificity)
- high positive LR important to diagnose

negative: (1 - sensitivity) / specificity
- low negative LR important to rule out a disease (the smaller the better)

Question 21

how do you calculate the post-test probability of disease?

Answer 21

pretest probabability X positive likelihood ratio

Question 22

which of the following is most useful for evaluating the probability of glaucoma in a glaucoma suspect: ROC, likelihood ratio, sensitivity, specificity

Answer 22

likelihood ratio. the other tools are binary (either you have glaucoma or you do not) and do not take into account pretest probability, which is important to consider in a glaucoma suspect

Question 23

RNFL is measured by OCT, and only for abnormal results a scanning laser perimetry is performed to measure RNFL. What type of study process is this, and what is a pitfall of this study design?

Answer 23

This is an example of using tests in series.
Pitfall: you may think you can multiple the original pretest probability by the product of the two likelihood ratios to get the final post-test probability, however the fact that these two tests are not independent is a confounder (they both measure RNFL, just by different mechanisms). Only tests without correlation can be performed reliably in series

Question 24

Type of study process: two tests performed at the same time, and a positive result of either one is considered diagnostic. Pitfall?

Answer 24

Tests in parallel. Pitfall: both tests must have high specificity for this to be useful

Question 25

calculate risk difference and relative risk:

case: 30% died
control: 70% died

Answer 25

difference = 40%, relative risk = 3/7

Question 26

calculation for odds and odds ratio

Answer 26

odds: ratio of favorable to unfavorable outcomes (odds of rolling a 5 or a 6 is 1/2)

odds ratio: odds of a subject with the disease having the exposure divided by the odds of the subject without the disease having the exposure

Question 27

what happens to the odds ratio as disease prevalence approaches zero?

Answer 27

odds ratio approaches relative risk

Question 28

what is the kappa statistic, and what cutoff value indicates an excellent kappa?

Answer 28

the degree of agreement between 2 or more individual methods or entities, accounting for agreement via chance alone. 75% and above is generally considered to be excellent