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

Sensitivity : (A) /(A+C) Susceptibility :( D) / (D+ B)

Disease

Positive

Disease

Negative

Test + A B

Test - C D

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

Positive predictive Value : (A) /(A+B) Negative predictive value :( D) / (D+ C)

Disease

Positive

Disease

Negative

Test + A B

Test - C D

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

Positive LR : (sensibility ) /(1-specificity) Negative LR :( 1-sensibility) / (specificity)

Disease

Positive

Disease

Negative

Test + A B

Test - C D

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

Relative Risk : A/B

<1 the exposure is protective

>1 the exposure is a risk factor

Exposure Nonexposed

Disease + A BNo disease C D

ARR/NNT

Relative risk reduction: A-B / (A) Absolute risk reduction :A-B ( in percent )

Number needed to treat (Favorite board question ) 1/ ARR

No treatment Treatment

Disease + A BNo disease C D

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 NNTA- 4B-20C-5D-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- 20B-30 C-50 D-5E- 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-15B-75C-6D-90E-we need to know the exact number of

patients in each group.

Classical Board Question

A metanalysis is just published an the benefits of N-acetylcystein 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 protectiveC-It is a riskD- 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