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UWorld > Biostatistics > Flashcards

Biostatistics Flashcards

1
Q

What are 3 principles of clinical trial design?

A

Physician-Patient relationship must exist, Physician must act in patient’s best interest, Written consent must be obtained from patient after full disclosure of clinical trial

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2
Q

When should median be used in place of mean?

A

When data is highly-skewed

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3
Q

Relationship between mean, median, mode in setting of negative skew?

A

Mean < Median < Mode

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4
Q

Relationship between mean, median, mode in setting of positive skew?

A

Mode < Median < Mean

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5
Q

Benefit of pooling data from several different studies in a meta-analysis?

A

Increases power of study … by increasing N

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6
Q

Describe the difference between effect modification and confounding?

A

Effect modification = SIGNIFICANT difference between strata; Confounding = NO significant difference between strata … (in confounding, the significance disappears)

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7
Q

Equation for Positive Predictive Value (PPV)?

A

A/(A+B) … TP/(TP+FP)

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8
Q

Definition of PPV?

A

Probability that patient actually has a disease, given a (+) test result

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9
Q

Observer bias typically occurs in which setting?

A

Absence of blinding

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10
Q

Observer bias is particularly likely when the studied outcome is …

A

Qualitative

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11
Q

Evaluation of multiple endpoints during a study increases the risk of …

A

Type 1 error

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12
Q

Describe a Type 1 error?

A

Reject the null, when the null was correct … (say there’s a difference, when there really wasn’t)

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13
Q

___ refers to increased likelihood of making a Type 1error when evaluating multiple end points?

A

Multiplicity

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14
Q

Define net clinical benefit?

A

Measure of a medication’s possible benefit, minus its possible harm

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15
Q

What is the most important aspect of a study, according to the Intention To Treat (ITT) principle?

A

Randomization

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16
Q

Intention To Treat (ITT) principle attempts to avoid the effects of …

A

Crossover, Dropout

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17
Q

What is the goal of Phase 4 clinical trials?

A

Post-marketing surveillance to monitor safety of medications that have already been released on market

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18
Q

Why might some clinical trials fail to detect serious side effects of new medications before Phase 4?

A

Inadequate power

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19
Q

Calculations for Positive Predictive Value (PPV) requires knowledge of …

A

Prevalence

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20
Q

___ is an expression of sensitivity and specificity that can be used to assess the value of a diagnostic test, independent of prevalence

A

Likelihood ratio

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21
Q

Equation for (+) likelihood ratio?

A

Patient with disease testing (+) / Patient without disease testing (+) … A / B

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22
Q

Equation for (–) likelihood ratio?

A

Patient with disease testing (–) / Patient without disease testing (–) … C / D

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23
Q

Smaller LR corresponds to ___ disease prevalence

A

Less likely

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24
Q

Most valid measure of central location for normally distributed data?

A

Mean

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25
Q

Most valid measure of central location for date with skewed distribution?

A

Median

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26
Q

___ refers to bias in which working populations are generally healthier than general population, exhibit lower mortality rates

A

Healthy worker effect

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27
Q

Healthy worker effect is a type of ___ bias

A

Selection

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28
Q

Describe sensitivity?

A

Probability of detecting a disease if it is present

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29
Q

Area under curve (AUC) if a Reciever-Operating Characteristic (ROC) is a reflection of …

A

Diagnostic accuracy

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30
Q

Describe statistical power?

A

Ability to detect an association, if that association exists

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31
Q

Value of meta-analysis?

A

Increase power of study

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32
Q

Higher SN means fewer …

A

False (–)

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33
Q

Higher SP means fewer …

A

False (+)

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34
Q

Relationship between SP and PPV?

A

Higher SP will increase PPV

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35
Q

Description of length-time bias?

A

Survival benefits of a diagnostic test are overstated … due to detection of a disease that is benign, slowly-progressive

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36
Q

Description of lead-time bias?

A

Test diagnoses a disease earlier … time from diagnosis to death appears prolonged … appears like early detection improves survival … BUT actually, disease was just diagnosed EARLIER in course

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37
Q

Randomized clinical trials are designed to test ___, but not ___

A

Efficacy; AEs

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38
Q

___ occurs in a meta-analysis when studies with significant (+) results are included, but studies with (-) non-significant results are not included

A

Publication bias

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39
Q

Greatest source of bias in case-control studies?

A

Recall bias

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40
Q

Quadriplegic patient remarks that that 5 years in his current state are equivalent to 1 year in full health – what is he referring to?

A

Quality-adjusted life years (QALYs)

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41
Q

Quadriplegic patient remarks that that 5 years in his current state are equivalent to 1 year in full health – what is his Time Trade-Off?

A

1/5 = 0.2

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42
Q

A patient had full health until 30 yo, then developed disability with TTO = 0.2; What is QALY at 35 yo?

A

30(1) + 5(0.2) = 31

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43
Q

Question for calculating Disability-Adjusted Life Years (DALYs)?

A

Years of Life Lost (YLL) + Years Lived with Disability (YLD) = YLL + YLD

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44
Q

Description of a factorial study design?

A

Study design that utilizes 2+ interventions with ALL possible combinations of those interventions … in a study with 2 interventions, a factorial design has 4 possible groups … Treatment A, Treatment B, Treatments A+B, Placebo (neither)

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45
Q

Alternate name for factorial study design?

A

Fully-crossed study

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46
Q

Equation for NNT?

A

1/ARR (absolute risk reduction)

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47
Q

What is the NNT for control group rate = (8/1000) and treatment group rate (6/1000)?

A

1/ ((8/1000) – (6/1000)) = 1/(0.002) = 500

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48
Q

In which situation would you need a larger sample size to show a significant difference between 2 groups … decreasing power, increasing a from 0.01 to 0.05, showing difference between groups = 5 instead of 8, using 1-tailed test instead of 2-tailed test?

A

Showing difference between groups = 5 instead of 8

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49
Q

Does using a 1-tailed or 2-tailed test require MORE power (aka larger sample size)?

A

2-tailed test

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50
Q

___ refers to repeating primary analysis calculations in a study by modifying certain criteria or variable ranges to determine whether those modifications significantly affect the results initially obtained

A

Sensitivity analysis

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51
Q

___ refers to bias that occurs when test A diagnoses a disease earlier than test B; As a result, the time from initial diagnosis to death appears prolonged with test A, although there is no actual improvement in survival

A

Lead-Time Bias

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52
Q

Recall bias is also known as a misclassification of ___

A

Exposure

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53
Q

Equation for attributable risk percent (ARP)?

A

[(Risk in exposed) – (Risk in unexposed)] / (Risk in total population)

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54
Q

Prevalence of smoking in a population is 50%; 5-year risk of ischemic CVA is 1:1000 in smokers, 0.5:1000 in non-smokers – what is the attributable risk percent of smoking on ischemic CVA?

A

(0.001-0.0005)/[(0.001.50) + (0.0005.50)] = 33%

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55
Q

Equation for Absolute Risk Reduction?

A

ARR = control rate – treatment rate

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56
Q

Equation for Relative Risk Reduction?

A

RRR = ARR/control rate

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57
Q

Equation for Relative Risk?

A

RR = treatment rate/control rate

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58
Q

Equation for NNT?

A

NNT = 1/ARR

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59
Q

Definition of positive likelihood ratio?

A

(probability of patient with disease testing +) / (probability of patient without disease testing +)

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60
Q

Definition of negative likelihood ratio?

A

(probability of patient with disease testing –) / (probability of patient without disease testing –)

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61
Q

Relationship between likelihood ratios and prevalence of disease?

A

LRs are independent of prevalence

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62
Q

Equation for Positive LR?

A

Positive LR = SN/(1-SP)

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63
Q

Equation for Negative LR?

A

Negative LR = (1-SN)/SP

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64
Q

LR < 1 indicates …

A

Decreased likelihood that disease is present

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65
Q

LR = 1 indicates …

A

No change in likelihood of disease

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66
Q

LR > 1 indicates …

A

Increased likelihood that disease is present

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67
Q

Main measure of association for case-control studies?

A

OR

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68
Q

Main measure of association for cohort studies?

A

RR

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69
Q

Main measure of association for cross-sectional studies?

A

OR

70
Q

How can you tell if a difference between 2 groups is significant, looking at standard error bars?

A

Overlap of standard error bars = non-significant difference

71
Q

Definition of Hazard Ratio?

A

Likelihood of an event occurring in a treatment group relative to a control group

72
Q

What is the null value for Hazard Ratio?

A

1

73
Q

Interpretation of Hazard Ratio < 1.0?

A

Event is less likely to occur in a treatment group vs. control group

74
Q

Interpretation of Hazard Ratio > 1.0?

A

Event is more likely to occur in a treatment group vs. control group

75
Q

Value of multiple linear regress?

A

Evaluate association between 1 quantitative dependent variable and 2+ independent variables (either quantitative or qualitative)

76
Q

How can you make a confidence interval more narrow?

A

Increase the power (sample size)

77
Q

Asymmetry in a funnel plot suggests …

A

Publication bias

78
Q

Utility of the ANOVA test?

A

Comparing mean value of continuous variable across several (3+) groups

79
Q

Which statistic does ANOV test generate?

A

F statistic … which is then used to calculate a p-value

80
Q

___% of data in a normal distribution lies within 1 standard deviation

A

68

81
Q

___% of data in a normal distribution lies within 2 standard deviations

A

95

82
Q

___% of data in a normal distribution lies within 3 standard deviations

A

99.7

83
Q

Relationship between mean, median, mode in a R-skewed distribution?

A

Mode < Median < Mean

84
Q

___ refers to probability of an individual testing positive given the presence of disease

A

Sensitivity

85
Q

___ refers to probability of an individual testing negative given the absence of disease

A

Specificity

86
Q

___ refers to probability that an individual who tests positive for disease actually has the disease

A

Positive Predictive Value

87
Q

___ refers to probability that an individual who tests negative for disease actually does not have the disease

A

Negative Predictive Value

88
Q

___ refers to the probability of an individual with the disease testing positive divided by the probability of individual without the disease testing positive

A

Positive likelihood ratio

89
Q

___ refers to the probability of an individual with the disease testing negative divided by the probability of individual without the disease testing negative

A

Negative likelihood ratio

90
Q

Equation that relates Positive likelihood ratio to SN and SP?

A

LR+ = SN/(1-SP)

91
Q

Equation that relates Negative likelihood ratio to SN and SP?

A

LR- = (1-SN)/SP

92
Q

2 values that must be known to calculated the PPV and NPV?

A

Prevalence, Pre-test probability

93
Q

Relationship between sample size (n) and Type 1 error?

A

Type 1 error increases as sample size increases

94
Q

Equation for attributable risk percentage?

A

[(Risk exposed) - (Risk nonexposed)] / (Risk exposed)

95
Q

Equation for hazard ratio?

A

(Hazard rate in intervention group) / (Hazard rate in control group)

96
Q

RCT investigates anti-inflammatory intervention as treatment for neuropathic pain; Prelim results shows significant reduction in pain score among treatment group (p=0.06); Significant reduction in pain among non-obese subjects (0.01), but non-significant increase in pain scores among obese subjects (p=0.06) - what is best explanation for difference in finding between obese and non-obese subjects?

A

Effect modification

97
Q

Relationship between PPV and R (correction coefficient)?

A

Higher R, Higher PPV

98
Q

What is non-ABCD equation for RR?

A

(rate of outcome in group 1) / (rate of outcome in group 2)

99
Q

Measure of association for case-control studies?

A

OR

100
Q

Primary endpoint of study is 30% reduction in PTH after treatment with Drug X or Drug Y; Primary efficacy for Drug X = 74%, for Drug Y = 85.4% with p<0.001; What is an appropriate 95% confidence interval for this study?

A

11.4 must be included in CI and at center of CI; CI cannot include 0

101
Q

DHS investigates an outbreak of Gi illness; Total of 25 individuals with GI bug and 75 individuals without GI bug are studied; 96% of patients with GI bug ate a local restaurant 2-4 days prior, 4% of patients without GI bug ate at same restaurant - what is the study design here?

A

Case-Control

102
Q

Highly sensitive tests are useful for___; highly specific tests are useful for ___

A

Screening; Confirmation

103
Q

Measure of association for case-control studies?

A

OR

104
Q

Measure of association for cohort and RCT studies?

A

RR

105
Q

Description of a nested case-control study?

A

Case-control study that draws its cases and controls from people participating in a follow-up (cohort or RCT) study

106
Q

A screening test with high ___ is the best for early detection of breast CA in a population with stable incidence of disease

A

SN

107
Q

Group of researchers wants to investigate an acute outbreak of acute diarrhea; Approximately 50 people developed severe hemorrhagic diarrhea, thought to be related to local seafood; Which study design is most appropriate to test this hypothesis?

A

Case-control

108
Q

Difference between indications for case-control vs. cross-sectional studies?

A

Case-control = investigate outbreaks … Cross-sectional = compare prevalence

109
Q

Utility of case-control studies?

A

Investigate outbreaks

110
Q

Cross-sectional studies begin with …

A

Risk factor/exposure … investigate outbreaks

111
Q

Utility of cross-sectional studies?

A

Compare prevalence

112
Q

Which study design is helpful for studying rare exposures?

A

Cohort

113
Q

Equation for Standardized Mortality Ratio?

A

(Observed # of Deaths) / (Expected # of Deaths)

114
Q

Purpose of pre-clinical trials?

A

Explore if & how new treatment may work

115
Q

Study population of pre-clinical trials?

A

Laboratory and animal models

116
Q

Purpose of phase 1 clinical trials?

A

Evaluate safety

117
Q

Study population of phase 1 clinical trials?

A

Healthy subjects, small number

118
Q

Purpose of phase 2 clinical trials?

A

Explore efficacy (does it work?)

119
Q

Study population of phase 2 clinical trials?

A

Small number of affected subjects

120
Q

Purpose of phase 3 clinical trials?

A

Compare new treatment to current Standard of care

121
Q

Study population of phase 3 clinical trials?

A

Large number of affected subjects randomly assign to treatment or control

122
Q

Purpose of phase 4 clinical trials?

A

Identify rare and long-term adverse effects

123
Q

A study has power of 80%; what is the probabilty of finding a significant effect if none actually exists?

A

Type 1 error = alpha; alpha = P value

124
Q

Description of (+) LR?

A

(probability of diseased patient testing positive) / (probability of healthy patient testing positive)

125
Q

Description of (-) LR?

A

(probability of diseased patient testing negative) / (probability of healthy patient testing negative)

126
Q

Equation for (+) LR?

A

SN / (1-SP)

127
Q

Equation for (-) LR?

A

(1-SN)/SP

128
Q

What is the meaning of the kappa statistic?

A

Inter-rater reliability (concordance)

129
Q

11 mo female presents shortly after diagnosed with rare disease; Parents ask about factors that mayhave contributed to developing of disease - which study design would be best?

A

Case Control

130
Q

___ studies are best at identifying potential risk factors for rare diseases

A

Case Control

131
Q

Description of “per protocol” analysis?

A

Compares treatment groups in an RCT by including ONLY patients who strictly adhered to protocol

132
Q

___ refers to an analysis that compares intervention groups by including all subjects as initially allocated after randomization, regardless of what happens during the study period

A

Intention to Treat (ITT)

133
Q

Intention to Treat (ITT) analysis leads to a ___ estimate of intervention effect

A

Conservative

134
Q

The results of a Intention to Treat (ITT) analysis reflect the __ effect of intervention as intended in the population

A

Real

135
Q

___ refers to an analysis that compares intervention groups by including data only from subjects who complete the intervention originally allocated at time of randomization

A

Per protocol

136
Q

___ refers to an analysis in which subjects are evaluated based on intervention they received, rather than the intervention to which they were randomized

A

As-treated analysis

137
Q

Which type of analysis is used to help preserve the benefits of randomization in superiority trials?

A

Intention to Treat (ITT)

138
Q

When comparing performance measures of 2 different tests, which statistic can show that there is no significant difference between the tests?

A

Overlapping CIs

139
Q

Which type of screening test will identify individuals without disease?

A

Screening test with high SP and high NPV

140
Q

2 tests for heterogeneity?

A

Q statistic, I2 index

141
Q

Which result of Q statistic suggests no heterogeneity?

A

P > 0.05

142
Q

Which result of I2 index suggests mild heterogeneity?

A

25%

143
Q

Which result of I2 index suggests moderate heterogeneity?

A

50%

144
Q

Which result of I2 index suggests high heterogeneity?

A

75%

145
Q

What is an epidemiologic parameter that does not vary with disease prevalence and provides useful information for individual patients?

A

Likelihood ratio (LR)

146
Q

Equation for positive LR?

A

Positive LR = SN/(1-SP)

147
Q

Equation for negative LR?

A

Negative LR = (1-SN)/SP

148
Q

___ refers to type of measurement bias that occurs when a study uses gold standard testing selectively in order to confirm a positive (or negative) result of preliminary test

A

Verification bias

149
Q

Effect of confounding variable on statistical significance?

A

Difference disappears when patients are stratified (based on the confounding variable)

150
Q

Confounding threatens the ___ of a study

A

Internal validity

151
Q

An epidemiologic study looks at adverse events in 10,000 patients who underwent PCI; According to results, 2% of patients who received metal stents developed thrombosis; 40% of patients who developed stent thrombosis passed away – the risk of death in a patient who develops stent thrombosis is best described as …

A

Case fatality rate

152
Q

How would you describe mortality rate?

A

Likelihood in general population of dying from disease

153
Q

Equation for FPR?

A

FPR = 1 – SP

154
Q

Equation for FNR?

A

FNR = 1 – SN

155
Q

Definition of standardized incidence ratio?

A

Observed # of cases / Expected # of cases

156
Q

What is the value of standardized incidence ratio?

A

Determine if occurrence of disease in a small population is high or low compared to expected value derived from a larger comparison population

157
Q

___ refers to the probability of detecting a true difference between normal and pathologic groups

A

Power

158
Q

Equation for power?

A

1 - b

159
Q

Relationship between variability and power?

A

Lower the variability, higher the power

160
Q

Relationship between precision and power?

A

Higher the precision, higher the power

161
Q

Diagnostic test shows that 92% of all skin tissue samples with (+) result were cancerous, while 99% of all skin tissue samples with (-) result were normal – what is best interpretation of these findings?

A

92% is PPV, 99% is NPV

162
Q

If RR of an outcome in group A as compared to group B is x, then RR in group B compared to group A is …

A

1/x

163
Q

RR of SCD in low-dose HCTZ group as compared to high-dose HCTZ is 0.4; how much does high-dose HCTZ increase risk of SCD compared to low-dose group?

A

Increased by factor of 2.5

164
Q

SN of study is 50% (2/4), SP is 98% (394/400) – what is PPV?

A

25%

165
Q

Ecologic (correlational) studies are based on ___ level

A

Population

166
Q

___ refers to the mistake of applying population-level information to individual level

A

Ecologic fallacy … results of ecologic study cannot provide individual-level conclusions

167
Q

___ refers to misclassification of events that results from knowing exposure status of patient in RCT

A

Observer bias

168
Q

Best way to reduced observer bias?

A

Blinding

169
Q

Description of Hawthorne Effect?

A

Patients change behavior when they know they’re being studied

170
Q

Hawthorne Effect poses a threat to the ___ of a study

A

Internal validity

171
Q

Association between risk factor and disease is more likely to be causal if …

A

Strength increases as exposure level increases (dose response relationship)

172
Q

__Refers to the incidence rate of an event and intervention group divided by the incidence rate of the same event in the control group

A

Rate Ratio

UWorld (21 decks)

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