Statistics Questions asked in Surgery Exams

Questions on Statistics

Q1 All are true except:
a. Senisitivity tp/tp+fn

b. Specificity tn/tn+fn

c. Ppv tp/tp+fp

d. Npv tn/tn+fn

Ans 1 b

Specificity = True Negative / TN + FP

Q2) To Study the effect of an antiviral  drug, we note down the initial viral load and then after  a month repeat viral load. What is the   the most appropriate statistical test of significance to test the change in viral load

(a) Paired t-test
(b) Unpaired or independent t-test
(c) Analysis of variance
(d) Chi-square test

Ans a) Paired t test

paired t test is used When comparing the same person results after giving a drug or doing a surgery or any intervention

A paired t-test is used when we are interested in the difference between two variables for the same subject. Often the two variables are separated by time

Unpaired t test

The unpaired two-samples t-test is used to compare the mean of two independent groups.

For example, suppose that we have measured the weight of 100 individuals: 50 women (group A) and 50 men (group B). We want to know if the mean weight of women  is significantly different from that of men In this case, we have two unrelated (i.e., independent or unpaired) groups of samples. Therefore, it’s possible to use an independent t-test to evaluate whether the means are different.

Chi-square test

The basic idea behind the test is to compare the observed values in your data to the expected values that you would see if the null hypothesis is true.

To determine whether the association between two qualitative variables is statistically significant

Analysis of Variance ANOVA example

A group of psychiatric patients are trying three different therapies: counseling, medication and biofeedback. You want to see if one therapy is better than the others.


Important Statistics for Surgeons

A) Comparing  outcomes

Tests of Significance to compare two 

  1. Student t test - Also called two sample t test , compares mean of two Independent  groups. For this you require two mean values, two sample variance and total no. of objects in the sample


B) Sensitivity, Specificity , PPV and NPV

You will have to remember the formulas like in the olden times of PHysics and Maths

Make a 2X 2 table  TP is True positive and so on, 

                           A (TP)                                   B (FP)


                          C (FN)                                   D ( TN) 


Case Control vs Cohort Studies

In case control studies, patients with the disease of interest (diabetes) are selected from a population and compared with representative ( same age and sex) , non diseased individuals from the same population.

There are several advantages of case-control studies.


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