Biostatistics

Question 1

alpha

Answer 1

The selected threshold for rejecting the null hypothesis, typically a = 0.05

Question 2

p-value

Answer 2

calculated value from the data that is compared to the alpha (0.05)

Question 3

Confidence Interval

Answer 3

CI = 1 - a

p < 0.05 shows a “95% Confidence” that the conclusion from the results are correct

Question 4

Difference Data w/ Confidence Interval

Answer 4

Statistically Significant if CI Range does NOT INCLUDE ZERO (0)

95% Confident that the value lies within that range. If it includes ZERO, then there is NO difference (NOT SIGNIFICANT)

ex. Treatment = 46, Placebo = 8
Difference (95% CI) = 38 (18-58)

Question 5

Ratio Data w/ Confidence Interval

Answer 5

Ratio Data = Relative Risk, Odds Ratio, Hazard Ratio

Statistically Significant if CI Range does NOT INCLUDE ONE (1)

Question 6

Type 1 Error

Answer 6

False-Positives: null hypothesis was rejected, when it should have accepted

alpha = Type 1 Error

Ex. a = 0.5, if p < 0.5 then the probability of a Type 1 Error is < 0.5

Question 7

Type 2 Error

Answer 7

False-Negatives: null hypothesis was accepted, when it should have been rejected

beta = Type 2 Error; set by investigators (10-20%; 0.1-0.2)

Power Analysis used to determine sample size needed to detect a true difference (reduce Type 2 Error)

Question 8

Power

Answer 8

Probability that a test will REJECT the Null Hypothesis CORRECTLY (Power to AVOID a Type 2 Error)

Power = 1 - b

Question 9

Risk

Answer 9

in Treatment with ADR / # in Control with ADR

Question 10

Relative Risk

Answer 10

Tx Risk / Control Risk

RR > 1: Higher Risk in Tx
RR < 1: Lower Risk in Tx

“RR of Tx = 57%. Tx were 57% AS LIKELY to have ADR as Control”

Question 11

Relative Risk Reduction

Answer 11

How much Risk is REDUCED in the Treatment compared to Control

RRR = (Risk Control - Risk Tx) / Risk Control

RRR = 1 - RR

“RRR of Tx = 43%. Tx were 43% LESS LIKELY to have ADR as Control”

Question 12

Absolute Risk Reduction

Answer 12

ARR = % Risk Control - % Risk Treatment

“ARR = 12%. For every 100 patients Treated, 12 FEWER patients will have an ADR”

Question 13

Number Needed to Treat (NNT)

Answer 13

NNT for 1 patient to BENEFIT (avoid ADR)

NNT = 1 / (Risk Control - Risk Tx)

NNT = 1 / ARR

ROUND UP

“For every 9 patients Treated, 1 patient will BENEFIT (avoid ADR)”

Question 14

Number Needed to Harm (NNH)

Answer 14

NNH for 1 patient to have ADR

NNH = 1 / (Risk Control - Risk Tx) Absolute value

NNH = 1 / ARR Absolute value

ROUND DOWN

“1 additional case of ADR for every 90 patients taking Tx instead of Control”

Question 15

Odds Ratio

Answer 15

Odds of an Outcome occurring WITH Exposure, compared to Odds of an Outcome occurring WITHOUT Exposure (Retrospective)

Exposure / Outcome (+) / Outcome (-)
Present / A / B
Absent / C / D

A = (+) Outcome WITH Exposure
B = (-) Outcome WITH Exposure
C = (+) Outcome WITHOUT Exposure
D = (-) Outcome WITHOUT Exposure

OR = AD / BC

Question 16

Hazard Ratio

Answer 16

Used in Survival Analysis (similar to Relative Risk)

HR = % Treatment / % Control

Question 17

Odds Ratio & Hazard Ratio Interpretation

Answer 17

OR or HR > 1: Event Rate in Tx is HIGHER than Event Rate in Control

HR of 2 indicates there are TWICE as many ADRs in the Treatment Group

Question 18

Types of Statistical Tests: Continuous Data

Answer 18

Continuous Data = increase by same amount

1) Interval Data = Zero does not mean “Nothing” (Celsius)
2) Ratio Data = meaningful Zero (BP)

Types of Tests [T-tests]
1) One-sample t-test: One Sample compared to General Population

2) Paired t-test: One Sample used for pre/post-Tx (Patient is their own Control)
3) Student t-test: Two Samples (Tx vs Control)
4) ANOVA = 3 or more groups

Question 19

Types of Statistical Tests: Discrete (Categorical) Data

Answer 19

Discrete (Categorical) Data:

1) Nominal Data = gender, race, mortality
2) Ordinal Data = NYHA Class, Pain Scale

Types of Tests:
Comparing between Two Groups (Tx vs Control):
1) Chi-square Test
2) Fisher’s Exact Test

Question 20

Questions to ask when interpreting Sensitivity/Specificity:

Answer 20

1) If the result is POSITIVE, what is the probability of ACTUALLY having the disease?
2) If the result is NEGATIVE, what is the probability of TRUELY NOT having the disease?

Question 21

Sensitivity

Answer 21

True Positive: % a test identifies a patient WITH the Dx

Sensitivity = # Tested Positive / # True Positive

Question 22

Specificity

Answer 22

True Negative: % a test identifies a patient WITHOUT the Dx

Specificity = # Tested Negative / # True Negatives

Question 23

Intension-to-Treat analysis

Answer 23

Includes ALL patients in each group, even if the patient did NOT complete the trial according to study protocol

*Has real world estimate of treatment effect

Question 24

Per protocol analysis

Answer 24

Only for patients who COMPLETED according to study protocol

Question 25

Forest Plots

Answer 25

Confidence Intervals for:

1) Difference Data = NOT Significant if crosses ZERO
2) Ratio Data = NOT Significant if crosses ONE