Stats in surgery

Question 1

Assuming that the results are normally distributed, what percentage of values lie within two standard deviations of the mean ?

Answer 1

95%

Question 2

Properties of normal districbution

Answer 2

symmetrical i.e. Mean = mode = median
68.3% of values lie within 1 SD of the mean
95.4% of values lie within 2 SD of the mean
99.7% of values lie within 3 SD of the mean

Question 3

Types of audit

Answer 3

Financial - historical- fairness, accuracy, and reliability of financial data

Operational - future- primary sources of evidence are the operational policies and achievements

Departmental review - A current period analysis of administrative functions, to evaluate the adequacy of controls, safeguarding of assets, efficient use of resources, compliance with related laws, regulations and institutional policy and integrity of financial information.

Standards- Comparison of care or passage of care against set and widely agreed standards or outcomes.

Systems - Evaluation of processes occurring within an institution- If the problem is related to a breakdown in the system or technology infrastructure

Question 4

Which device would be most suited for resection of a parietal lobe tumour?

Answer 4

CUSA device

Question 5

How Is data from multiple RCT best displayed

Answer 5

Forest plot

Question 6

Who to report to if patient in a study experiences an adverse outcom

Answer 6

Ethics committee

Question 7

Calculating RR

Answer 7

Exp event rate
Control event rate

RR=EER/CER

Question 8

When to use bipolar over monopolar

Answer 8

Bipolar is most commonly used in operations of the digits/penis (to avoid monopolar current focused over a smaller region), in patients with pacemakers (to avoid electrical involvement with the pacemaker), or in microsurgery

Question 9

What does a p value represent

Answer 9

Risk of type 1 error

Question 10

Type 1 vs 2 error

Answer 10

Type 1 Error
the null hypothesis is rejected when it is true - i.e. Showing a difference between two groups when it doesn’t exist, a false positive
Rate of type 1 error is the given the value of α
It usually equals the significance level of a test

Type 2 Error
the null hypothesis is accepted when it is false - i.e. Failing to spot a difference when one really exists, a false negative.
Rate of type 2 errors is given the value of β
It is related to the power of the test

Question 11

Diathermy used in thyroid gland surgery

Answer 11

Bipolar to minimise risk to RLN

Question 12

What does a null hypothesis state

Answer 12

A null hypothesis (H0) states that two treatments are equally effective (and is hence negatively phrased). A significance test uses the sample data to assess how likely the null hypothesis is to be correct.

Question 13

Alternative hypothesis states

Answer 13

The alternative hypothesis (H1) is the opposite of the null hypothesis, i.e. There is a difference between the two treatments

Question 14

Incidence vs prevalence

Answer 14

The incidence is the number of new cases per population in a given time period.

The prevalence is the total number of cases per population at a particular point in time.

Question 15

Number needed to treat calculation

Answer 15

1/ARR

ARR= RT- RT2

Question 16

Case control vs cohort

Answer 16

Cohort - measure RR
Observational and prospective

Case control- Observational and retrospective
OR
Useful for studying rare conditions
Prone to confounding

Question 17

If a dataset requires multiple testing

Answer 17

Use Bonferroni correction

Question 18

Tests for not normal distributions

Answer 18

Chi Squared and Mann Whitney U tests

Sample size small- Fischer

Question 19

What has the biggest impact on PPV

Answer 19

Prevelance

Question 20

Definition of PPV< NPV, Sens, spec

Answer 20

The positive predictive value (PPV) is the probability that an individual with a positive screening result has the disease. The sensitivity is the probability that an individual with the disease is screened positive and the specificity is the probability that an individual without the disease is screened negative.
Its value depends upon the prevalence of the condition being tested for and the sensitivity of the test used.
It may be calculated by dividing the number of true positives by the number of true positives and the number of false positives. §]

Question 21

Levels of evidence

Answer 21

I - evidence from meta-analysis of randomised controlled trials
II - evidence from at least one well designed controlled trial
III - evidence from correlation and comparative studies or use of historical controls
IV - evidence from case series
V - Expert opinion or founded on basic principles

Question 22

Components for power calculation

Answer 22

Size of effect
Significance level
Sample size used to detect the effect
Desired power value

Question 23

Mean in relation to median with skew

Answer 23

In a left-skewed distribution, the mean is less than the median

Question 24

Child is unstable requires blood, parents refuse what do you do

Answer 24

Treat in emergencies

Question 25

Pre- and post- test odds and probability

Answer 25

Pre test prob= prevelencne Pre test odds= PreTP/1-PreTP Post test odds= PreTO x likelihood Post test probs= PostTO/1-PostTO

Question 26

Montgomery ruling

Answer 26

The doctor is therefore under a duty to take reasonable care to ensure that the patient is aware of any material risks involved in any recommended treatment, and of any reasonable alternative or variant treatments.

Question 27

Cluster trials

Answer 27

Groups are randomised rather than individuals Require increased recruitment to achieve same stat power as individual more prone to unit of analyses errors than individual based trial Analysis is more complex

Question 28

Intention to treat analysis

Answer 28

An intention to treat analysis considers data according to the treatment to which the patient was randomised, rather than the treatment which was recieved.

Question 29

Bolam test

Answer 29

decision made by a doctor is in agreement with the professional standard of medical practise.

Question 30

Consent forms

Answer 30

1- competent adults, consciousness may be impaired 2- adult on behalf of child 3- not impaired 4- adult lacking capacity

Question 31

When is a MCA advocate needed

Answer 31

Patient requires intervention Time pressure Lacks capacity for themselves a best interests decision is required because the doctor has assessed the person as not having the capacity to make the decision themselves the person does not have family or friends with whom it is appropriate to consult about the decision.

Question 32

What does r represent

Answer 32

Correlation coefficient

Question 33

Levels of evidence

Answer 33

1a- systematic review of RCTs 2- cohort 3- case- control 4- case series 5- expert opinion a- systematic review of b- individual

Question 34

p value of 0.05

Answer 34

<5% due to chance

Question 35

Friedman test use

Answer 35

3 or more Non parametrical- not normal distribution

Question 36

95% CI meaning

Answer 36

That 95% confidence that the true population value lie within this range

Question 37

Chi squared test vs t test

Answer 37

T test for continuous data Chi squared for categorical - they either did or they didnt