5: Evidence based medicine 1

Question 1

What is a randomised control trial?

Answer 1

Comparison of intervention vs control in different randomised groups of patients

Question 2

What is intention to treat analysis?

Answer 2

ALL patient’s outcomes are included in the study after randomisation

e.g patients who are killed crossing the street to collect the drug

Question 3

Why is intention to treat analysis important?

Answer 3

Preserves randomisation

Question 4

What is per protocol analysis?

Answer 4

Only patients who received the intervention / control are studied

e.g patients who had a stroke BEFORE cerebrovascular surgery wouldn’t be included in results

Question 5

What’s wrong with per protocol analysis?

Answer 5

Randomisation lost

specific factors will have resulted in those events occuring in the patients and you’re basically ignoring them

Question 6

What is the term used to describe the chances of an event occuring?

Answer 6

Probability

Question 7

What are two ways of expressing probability in a study?

Answer 7

P value

Confidence interval

Question 8

What is a p value?

Answer 8

Numerical value indicating the probability that an observation has occured due to chance

Question 9

What does due to chance mean?

Answer 9

The observation is not statistically significant

Question 10

What does statistically significant mean?

Answer 10

An observation which is probably not due to chance

Question 11

What does a p value of 0.0014 mean?

Answer 11

14 observations out of 1000 will be due to chance; statistically insignificant

Question 12

What is a confidence interval?

Answer 12

Range of values which probably contains the true value

Question 13

What is the null hypothesis?

Answer 13

There’s no difference between the two groups; observations are likely due to sampling error

Question 14

A small p value (< 0.05) indicates that the null hypothesis is (true / false).

Answer 14

Small p value means FEW observations are due to chance

So the null hypothesis is probably FALSE

Question 15

A large p value (> 0.05) indicates that the null hypothesis is (true / false).

Answer 15

Large number of observations are due to chance

So null hypothesis is probably TRUE

Question 16

What is a Type 1 error?

Answer 16

False positive

You falsely reject the null hypothesis when it is true

Question 17

What does a p value of 0.05 mean in terms of Type 1 error?

Answer 17

1 in 20 times you reject the null hypothesis when it’s true

Question 18

What is a Type 2 error?

Answer 18

False negative

You accept the null hypothesis when it’s false

Question 19

A Type 1 error is also known as a false ___.

Answer 19

False positive

Question 20

A Type 2 error is also known as a false ___.

Answer 20

false negative

Question 21

What is a confidence interval?

Answer 21

Range of values which is likely to contain the true value

Question 22

What does a 95% confidence interval mean?

Answer 22

95% of true values are probably within the confidence interval

Question 23

What are three possible explanations for a positive result?

Answer 23

Null hypothesis is false

Random chance

Confounding variable

Question 24

What are four possible explanations for a negative result?

Answer 24

Null hypothesis is true

Random chance

Confounding variable

Size of the effect is too small to measure

Question 25

What is statistical power?

Answer 25

Probability of correctly rejecting the null hypothesis

Question 26

What affects the statistical power of a study?

Answer 26

Size of effect being measured

Sample size of study