Medical Statistics

Question 1

Standard deviation

Standard error

Answer 1

Standard deviation - describes the spread of data around the mean

Standard error - standard deviation of the sample mean

Question 2

Confidence interval (CI)

Answer 2

Measures the uncertainty in measurement .
Confidence interval gives the range in which the true mean value is likely to be.
95% CI = range in which 95% of the population lies.
0 is not significant eg 95% CI -5 to +30 change in blood pressure after anti hypertensives means that more than 5% chance that there is no change in BP.
The size of CI is related to sample size - larger samples have smaller CI ( smaller range)

Question 3

Prevalence

Incidence

Answer 3

Prevalence - the proportion of population with the disease in a time point

Incidence - the rate of new onset disease during a period of time

Question 4

Odds

Odds ratio

Answer 4

Odds - the number of times an event is likely to occur / the number of times it is unlikely to occur
Odds ratio - odds of the disorder in the experimental group / odds of the disorder in the controlled group

Question 5

Risk 
Risk ratio (RR)

Answer 5

Risk is the probability that the event will happen. n
Risk = the number if events that is likely to happen/ total number of events
Risk ratio = risk of an event in experimental group / risk of event in control group

Question 6

What is the difference between standard deviation and confidence interval?

Answer 6

Standard deviation tells us about the spread (variability) of the data in a sample and the CI tells us the range in which the true value ( the mean if the sample is infinitely large) is likely to be.

Question 7

What is the P - value?

Answer 7

P value is the probability that the result is due to chance or probability that the results given a true null hypothesis.

P = 0.05 means that the difference in result happening by chance is 1 in 20
Threshold of statically significance.

Question 8

What is the difference between statically significant and clinical relevance?

Answer 8

If a study is too small, the results are unlikely to be statically significant even if the intervention actually works.

Large studies may find a statically significant difference that is too small ti have any clinical significance/relevance.

Question 9

Number to treat

Answer 9

Number of patients required to be treated for 1 patient to gain a benefit.

Question 10

Type 1 error

Type 2 error

Answer 10

Type 1 error - false positive
rejecting the null hypothesis when it is true
due to bias and confounding factors

Type 2 error - false negative
accepting the null hypothesis when it is false
due to small sample size

Question 11

Intention to treat analysis

Answer 11

To include ALL the participants data regardless on whether they finished the study. This decrease attrition bias.
Drop outs increase Type 1 and 2 error.

Question 12

Sensitivity

Specificity

Answer 12

Sensitivity - true positive
Patients who has the disease and is tested positive

Specificity - true negative
Patients who does not have the disease and is tested negative

Question 13

What is the POWER of a study?

Answer 13

Power of the study is the ability of the study to find the difference between the arms.

Power of 0.8 - 80% chance for the study to find a difference.
Larger the sample size and larger the power and smaller type 2 error
Power = 1 - Type 2 error ( usually 0.2)

Question 14

What is a parametric test?

Answer 14

Parametric test are used to compare samples of normal distribution.
(samples that follow a specific distribution)
eg ANOVA and students T test

Question 15

What is ANOVA?

Answer 15

This is used to compare the means of 2 or more samples to see whether they come from the same population.
( used in 2 or more samples + parametric)

Question 16

What is student T test?

Answer 16

This is used to compare 1-2 samples. They test the probability that the samples come from the same population with the same mean.
( 1-2 samples and parametric)

Question 17

Tell me a few types of bias.

Answer 17

Selection bias - sampling bias and response bias
Observation bias - failure to measure factors/outcomes accurately
Attrition bias - number of drop outs differs in separate arms, those who are left are not representative of the original study
Confounding factors - associated with exposure and outcome but independent. See stratification

Question 18

What is stratification?

Answer 18

This is way of randomisation - by separating the confounding factors evenly in groups before randomising the groups.
Other methods of randomisation - simple, block and stratified

Question 19

Tell me 5 points of cohort study.

Answer 19

1. observation study on a group of ppl without the disease over a period of time to establish the incidence of the condition
2. can be prospective/ retrospective
3. can be more influence by confounding factor/ more vulnerable to bias
4. good at looking at risk factors
5. different study design to others - as it has risk factors then developed outcome

Question 20

Advantage and disadvantage in cohort

Answer 20

Advantage
1 good at looking at risk factors, latent periods and uncommon disease
2 multiple outcomes ( the million women study)

Disadvantage
1 need large sample size
2 long follow up period
3 may be lost to drop outs

Question 21

What is CONSORT statement?

Answer 21

1 consolidated standards of reporting tools
2 1996 , last one 2010, comprises 25 item checklist and Consort diagram
3 guide researchers to writing RCT and help evaluate such studies

Question 22

What is PRISMA statement?

Answer 22

1 preferred reporting items of systemic review and meta analysis
2 guide researchers to write systemic reviews and meta analysis
3 item checklist and flow diagram

Question 23

What is an impact factor (IF)?

Answer 23

• It is an objective measure of a journal’s relative importance in its field.
• Calculated each year based on a 3 year period
• published in journal of citation reports.
• Average number of times published papers are cited in the last 3 years up to 2 years after publication.

Question 24

Levels of evidence - Tell me about Oxford Taxonomy

Answer 24

1a Systemic review of RCT
1b High quality RCT
1c RCT
2a Systemic review of cohort studies
2b Cohort study 
2c Outcome research
3a Systemic review of case control studies
3b Case control study
4  Case series
5  Expert opinion

Question 25

Mode
Median
Mean

Answer 25

Mode - most common value of the data set
Median - middle value of the data set
Mean - sum of all values / number of values in the data set

Question 26

What is a meta-analysis?

Answer 26

A meta-analysis is the use of statistical methods to combine results of individual studies, usually RCTs.
It allows researchers to have a more precise estimation of treatment effect.

Question 27

What is a systemic review?

Answer 27

This is a secondary research study in which a literature review is conducted according to a strict protocol to identify all studies on the subject.
( Need to conduct under PRISMA guidelines.
Strict inclusion and exclusion criteria to ensure decreased bias.)

Question 28

What are the advantages and disadvantages of meta-analysis?

Answer 28

Advantage

1. Greater statistical power to detect an effect than a single study
2. Combine results from several studies hence less likely to be influenced by local factors.

Disadvantages

1. pooling results from several studies may not predict the results of a single large study.
2. if the source is poorly designed, the meta analysis will produce unreliable results.

Question 29

What is a Forest plot?

Answer 29

1. It is used to demonstrate the results of meta analysis
2. vertical line of no effect
3. Diamond = point estimate of pooled result
4. Horizontal line - 95% CI; if CI crosses the vertical line of no effect, either sample size to too small or the result is not statically significant.

Question 30

What is a Randomised control trial?

Answer 30

1. Allows comparison of 2 or more possible interventions
2. Participants are randomly divided to separate arms

Advantage
Minimise the risk of allocation bias
Can be combined into systemic reviews/meta analysis to produce high level evidence

Disadvantage
Time and cost
Difficult on long term FU
Usually focus on a narrow area of patients condition

Question 31

What are the different phases of RCT?

Answer 31

1. Healthy people
2. Relevant illness
3. Clinical setting
4. Postmarketing surveillance

Question 32

What is allocation concealment?

Answer 32

A method used to prevent clinicians working out the trial allocation of the next participant - including sealed envelopes, central randomisation.

(Blinding - the clinician, patient and outcome assessors are unaware on which intervention)

Question 33

What is regression analysis?

Answer 33

It aims to model the relationship between multiple variables.
It is usually used to identify and control the effects of possible confounding variables in an experiment.
( in order for it to be adequately powered, it need at least 10 patients for each variable in the model : if there are 10 variables, need at least 1000 pts)

Question 34

What are ROC curves - receiver operating characteristic curves?

Answer 34

It shows the performance of a test.
Sensitivity Y, Specificity X
Upper left corner - best test
If its a diagonal line - the test has no significance.
The area of the graph - summarise the performance of the test.

Question 35

What is survival analysis?

Answer 35

It compares time to event data.
Uses Kaplan Meier curve - shows event rates over a time period
( uses a log rank test)
Censoring - accounts for missing data