Transcript Statistics for the board

Statistics for the board
September 14 th 2007
Jean-Sebastien Rachoin MD
 Rule #1 : Statistics is cool
 Rule # 2 : Statistics is easy
 Rule # 3: P value is not every thing
 Statistics for the board
 Statistics questions : 2 -3 / modules
 Cardiology question 31 total questions
 GI questions 28 questions
 Heme-onc questions 27 questions
For the board
Disease
Positive
Disease
Negative
Test +
A
B
Test -
C
D
 Sensitivity : (A) /(A+C)
 Susceptibility :( D) / (D+ B)
Classical Board question
 A new screening test is being developed by
Amgar . Inc for early detection of CKD.
Which of the following assays would be
preferable?
A- Sensibility 90 % Specificity 20 %
B-Sensibility 80 % Specificity 50%
C-Sensibility 70 % Specificity 60 %
D-Sensibility 60 % Specificity 90 %
 Amgar inc. is developing a test to confirm HIV
status in patients with a positive ELISA test.
Whish assay should be used?
A- Sensibility 90 % Specificity 20 %
B-Sensibility 80 % Specificity 50%
C-Sensibility 70 % Specificity 60 %
D-Sensibility 60 % Specificity 90 %
 A few years ago the AUA decided to change the
threshold for Abnormal PSA value from >10 to >4 .
 What happened to the sensibility and the specificity
of the test?
A-They were not affected
B-They both increased
C-The specificity increased and the sensibility increased
D- We need the prevalence of the disease
E-The sensibility increased and the specificity
decreased.
 Screening test : high sensibility
 Confirmatory test : high specifity.
For the board
Disease
Positive
Disease
Negative
Test +
A
B
Test -
C
D
 Positive predictive Value : (A) /(A+B)
 Negative predictive value :( D) / (D+ C)
Classical Board question :
 As the prevalence of a disease decreases in a
population what happens to the Positive predictive
value:
A- Stays the same and the Sensibility decreases
B-Increases and the Negative predictive values
decreases
C-Decreases and the Negative predictive value
increases
D-Neither the Positive or the negative predictive values
are affected
 A 40 year-old female wants to have a mammogram.
The prevalence of Breast cancer in her population is
estimated to 20 in 1000. The sensibility of the
mammogram is 70 % and the specificity is 90 % . If
her mammogram is positive what is the chance she
has breast cancer?
A-2%
B-12%
C-22%
D-25%
E-42%
For the board
Disease
Positive
Disease
Negative
Test +
A
B
Test -
C
D
 Positive LR : (sensibility ) /(1-specificity)
 Negative LR :( 1-sensibility) / (specificity)
 LR are used to assess the pre and post test
probability of the presence of a disease.
 Post-test Probability = LR X Odds
 Always remember Odds is not Probability:
 Probability = Odds /(Odds+1)
 Odds =Probability /(1-Probability)
 Step 1-A test has a sensibility of 60 % and a
specificity of 90 % what is the Positive LR ?
A-6
B-10
C-20
D-60
E-can not calculate need more information
 Step 2: the pretest probability of a disease is
20 % what are the odds of its occurrence?
A-0.10
B-0.20
C-0.25
D-0.70
What is the most compelling physical finding for the
DIAGNOSIS of DVT in a patient?
Sensitivity
Specificity
Erythema
20%
95 %
Swelling
70%
70%
Pain
2%
99%
SOB
4%
80%
Relative risk
Disease +
No disease
Exposure
Nonexposed
A
C
B
D
 Relative Risk : A/B
<1 the exposure is protective
>1 the exposure is a risk factor
ARR/NNT
Disease +
No disease
No treatment
Treatment
A
C
B
D
 Relative risk reduction: A-B / (A)
 Absolute risk reduction :A-B ( in percent )
 Number needed to treat (Favorite board question )
1/ ARR
Classical Board question
 A new treatment for terminal heart failure is
tried in a phase III trial . Mortality at 2 years is
60 % with traditional medication and 35 %
with this new drug .
What is the NNT?
A-4
B-5
C-25
D-35
Classical Board question
 A new experimental chemotheray drug is
being tested for metastatic pancreatic
adenocarcinoma. Patient who receive the
drug have a 35 % survival rate at 1 year
those who don’t have a 15 % survival rate.
What is the NNT
A- 4
B-20
C-5
D-15
Classical Board question
A new treatment for breast cancer is being tested . In
Population A 180 receives the treatment and 72
survive at 5 years . Of the 240 patients who do not
receive the treatment 48 survive .
What is the NNT for this treatment?
A- 20
B-30
C-50
D-5
E- need more information can not calculate.
300 subjects are randomized to trial for a drug .
Placebo group had a mortality of 90%.
Treatment group 75 % . How many patients
need to be treated to save a life?
A-15
B-75
C-6
D-90
E-we need to know the exact number of
patients in each group.
Classical Board Question
A metanalysis is just published an the benefits of Nacetylcystein in patients undergoing cardiac
catheterisation . The risk of developping Contrast
nephropathy with NAC is found to be 0.7 [ 0.5-1.1] p
value : 0.05
What can you say about the effect of NAC and
cardiac catheterisation?
A- There is no proven effect
B- it is protective
C-It is a risk
D- We need more information on the type of contrast
used
What about p?
 “ P” is not a magical number
 P tells you whether or not an observed effect
can be explained by chance .
 It measures the probability that the results
due to chance alone .
Concept of CI
 Confidence interval (CI) is the interval whithin
which exists a high probability of finding the
true value .
 Eg : Odds-ratio : 2.0 (CI 95 % : [1.5-3])
 On a question with a CI that encompasses 1
for a risk or 0 for an effect there is no
statistically significant relationship even is p
<0.05.
Important type of studies
 Observational /Retrospective
 For rare disease : case-series.
 Intent to treat : is a method of adjusting for
bias caused by participants leaving the study
because of treatment.
 Randomized – controlled trials
 Multicenter trial refers to the number of
centers not the type of study.
 Systematic review (Cochrane)
 Meta-analysis: quantitative combination of
multiple studies.
 The best level of evidence is Systematic
reviews or RCT.
 Evidence of Class A or I is considered to be
supported by good EBM evidence
 Evidence of class D or IV : expert opinion is
BOGST
Thank You