statistics 1

Question 1

what is a randomised control trial?

Answer 1

when a study population is chosen and sampled and then the groups are randomly split in two one groupie given the treatment and another is given the control both groups are followed up and the results are compared

Question 2

why are trials conducted?

Answer 2

• ascertain the safety of a new drug
• demonstrate tolerability
• monitor adverse effects
• demonstrate efficacy
• to see if it is superior to an equivalent treatment
• to demonstrate cost effectiveness

Question 3

what is the benefit of randomising in a trial?

Answer 3

• allows random confounding factors to be held constant between 2 groups
• all potential confounding variables (known and unknown) will be distributed by chance between two groups
• it is the only way to infer causality between groups

Question 4

what is a confounder?

Answer 4

a variable that affects both the dependent and independent variable

e.g. when investigating how maternal weight gain affects infant birthweight- the length of gestation also influences both of these factors, therefore it is a confounding factor

Question 5

what is equipoise?

Answer 5

Clinical equipoise is the assumption that there is not one ‘better’ intervention present (for either the control or experimental group) during the design of a randomized controlled trial (RCT).

It is not ethically acceptable to randomize patients to an arm of a trial which is known to be inferior or potentially harmful.

Question 6

what is internal vs external validity

Answer 6

internal validity relates to how well a study is conducted (its structure) and whether the study establishes trustworthy cause and effect

external validity relates to how applicable the findings are to the real world…. to what extent can these findings be generalised to other people, situations and times?

Question 7

what is the definition of bias?

Answer 7

any departure of results from the truth

• a partiality that prevents objective consideration of the issue or situation

in RCT:

systematic error is introduced into sampling or testing by selecting or encouraging one outcome or answer over others

bias is independent of both sample size and statistical significance

Question 8

what are all the different types of bias

Answer 8

• selection bias
• performance bias
• attrition bias
• observer/detection bias

Question 9

what is selection bias?

Answer 9

• Study sample does not represent target population
• Can happen when participants are asked to volunteer for a study

* can also happen when participants are recruited from a particular area….e.g. patients being trailed for a new diabetes drug recruited from a hospital setting may be different from the diabetic population at large

Question 10

what is observer/detection bias?

Answer 10

• can occur in 2 ways
  1. not adequately capturing the outcome f interest
• e.g. when measuring disability relating to schizophrenia, how do you account for death? it at all?
  2. if there is a systematic difference in the way that information is being collected for the different groups being studied
• e.g. using manual and electric blood pressure measurements

Question 11

what is attrition bias?

Answer 11

Attrition bias is a systematic error caused by unequal loss of participants from a randomized controlled trial (RCT)

Question 12

intention to treat analysis?

Answer 12

method for analyzing results in a prospective randomized study

• all participants who are randomized are included in the statistical analysis and analyzed according to the group they were originally assigned

regardless of what treatment (if any) they received. this helps to erradicate attrition bias

Question 13

per-protocol analysis?

Answer 13

only analyse patients who finish the treatment according to the study protocol.

so it does not include people who have dropped out of the study but this can lead to bias

as When some participants in either or both the groups are excluded, the remaining participants in the two groups can no longer be considered as balanced

Question 14

what is a type 1 error?

Answer 14

false positive- rejection of a true null hypothesis

Question 15

what is a type 2 error?

Answer 15

false negative-non-rejection of a false null hypothesis

Question 16

what is a P value?

Answer 16

This is the probability of the null hypothesis being true p = or < 0.05 is often taken as the threshold to reject the null hypothesis (5 in 100 / 1 in 20 chance of the result being due to chance)

Question 17

what is the relationship between sample size for trials and error in estimation?

Answer 17

error in estimation decreases as sample size increases

Question 18

what are the different categorical catagories?

Answer 18

1. yes/ no questions 2. answers with several catagories that don’t have an order…e.g. sexuality-> straight, bisexual, gay, trans 3. an answer with several ordered levels such as economic status-> rich, okay, poor

Question 19

what is a variable scale?

Answer 19

continuous scale such as height

Question 20

what is parametric data?

Answer 20

it makes the assumption that the data is evenly distributed

Question 21

what is the problem when the data is skewed?

Answer 21

• the mean is different from the median - the mean is greatly influenced by the skew - so the median is a better representation of the data than the mean, and you should give the inter quarter rang instead of the standard deviation

Question 22

how do we descriptively analyse scale data?

Answer 22

mean with standard deviation or median with interquartile range

Question 23

how do you descriptively analyse categorical data?

Answer 23

use percentages

Question 24

when to use T- test or ANOVA?

Answer 24

For randomised control trials that are comparing a continuous variable between groups

e.g. if asking is one group gets better scores on a SCALE than another= does drug A reduce depression symptoms compared to drug B?

the population should be randomly sampled

the observations should be made independently

and variances in each group should be equal

variables should be normally distrbuted

if more than 2 groups are being compared use the ANOVA test

Question 25

when to use chi squared test?

Answer 25

when comparing if one group has a higher categorical outcome than another? e.g. is drug A associated with fewer people meeting depression criteria than drug B

Question 26

what type of test do you use for parametric data?

Answer 26

T test

Question 27

what are the non parametric tests?

Answer 27

mann-whitney U wilcoxon rank sum kruskal wallis spearman’s rho

Question 28

how is biased defined in statistics?

Answer 28

a tendency of an estimate to deviate in one direction from the true value

Question 29

in what types pf trials can blinding be easily implemented?

Answer 29

drug trials

Question 30

inn which types of trials can blinding nt be as easily implemented?

Answer 30

• surgery vs non surgery
• psychological interventions

Question 31

what type of blinding can you implement for trials involving surgery or psychological interventions which are difficult to blind in the conventional way?

Answer 31

• blnd the patient to the hypothesis
• making it a hidden superiority trial

Question 32

what is the poer of a study- and what level power studies are accepted?

Answer 32

The statistical power of a study is the power, or ability, of a study to detect a difference if a difference really exists

• typically accepted at 80-90%

Question 33

what are the 3 different types of blinding?

Answer 33

• single blinding= only the patients are blinded
• double blinding= blinding of the patients and the physicians
• tripple blinding= blinding of patients, physicians and study investigators

Question 34

why is it important to have an adequate sample size in an investigation?

Answer 34

too few= if yo get a null result you cant be sure it its because there is actually no effect or if there is simply no evidence of an effect

too many= its unethical to involve too many people, you shouls involve just enough to find an effect. it is also too expensive

Question 35

what an alternative test t the t test for comparing non-parametrically distributed data for 2 groups?

Answer 35

Mann whitney U