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 + AB Test - CD

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 + AB Test - CD

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 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 + AB Test - CD

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? SensitivitySpecificity Erythema20%95 % Swelling70% Pain2%99% SOB4%80%

Relative risk Relative Risk : A/B <1 the exposure is protective >1 the exposure is a risk factor ExposureNonexposed Disease + AB No disease CD

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 treatmentTreatment Disease + AB No disease CD

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 N- acetylcystein in patients undergoing cardiac catheterisation. The risk of developping Contrast nephropathy with NAC is found to be 0.7 [ ] 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