RCTs and means

Question 1

Define RCT

Answer 1

A study in which participants are randomly allocated between a treatment intervention and a controlled group.

Question 2

Confounder

Answer 2

A variable that influences both dependent and independent variable, causing a false association

RCTs ensure that confounding are distributed by chance.

Question 3

Clinical Equipoise

Answer 3

Genuine uncertainty as to whether one treatment will be more beneficial than another when assigning patients to different treatments of a clinical trial.

It would be unethical to give patients a treatment that is harmful or inferior

Question 4

Bias

Answer 4

A departure of results from the truth due to a systematic error in sampling/ testing by selecting/encourage one outcome.

It is independent from sample size and statistical significance.

Question 5

Causes of bias

Answer 5

Instrument that measures outcome. May not represent true value.

Choice of participants.
- May not be representative of population

Influence of observers or researchers.
- May change measurements for different groups.

Question 6

Selection bias

Answer 6

Occurs when study sample does not represent target population.

Examples: recruiting from volunteers who may have certain incentives, recruiting from inpatients who represent the more ‘ill’ population of a condition.

Question 7

Observer bias

Answer 7

Systematic difference between the way information is collected and how the group is being studied.

Question 8

Blinding

Answer 8

Concealing the group allocation from one or more individuals involved.

Question 9

Pros of blinding

Answer 9

Reduces selection bias by preventing the different treatment or assessments of groups

Question 10

Types of trials that can be blinded

Answer 10

Drug trial with placebo

Sham surgery (during everything you would do in a normal surgical intervention, without doing the actual therapeutic step) vs other intervention

Question 11

Types of trials that cannot be blinded

Answer 11

Psychological interventions

Surgical vs non-surgical trial

Question 12

Types of RCT

• Open
• Single-blinded
• Double blinded
• Triple blinded

Answer 12

Open
- Everyone in trial knowns

Single
- patients are blinded

Double
- patients and treating physicians blinded

Triple
- Patients, treating physicians and study investigators blinded

Question 13

Type 1 and type 2 error

Answer 13

Type 1

• When there is an observed difference that has no true difference
• Significance level accepted at 5%.

Type 2
- When there is no observed difference but there is a true difference

Question 14

Types of data

• Categorical
• Scale variables
• Qualitative

Answer 14

Categorical

• nominal, binary, dichotomous
• E.g sex, social class, ethnicity

Scale variables

• continuous, interval
• Age, height, weight

Qualitative
- Text, words

Question 15

Data skew

Answer 15

When there is more count in the tail than expected in normal distribution.

Positive- tail on right
Negative- tail on left

Problems

• Mean is difference from median
• median is better representation of data than mean as mean is highly influenced by tail.

Question 16

Prevalence

• Definition
• Equation

Answer 16

Frequency of cases in a given population at a designated time:
- diagnosed or diagnosed + receiving treatment/ management

Expressed as percentage/ proportion

Equation

• No. of people with diagnosis / total population.
• usually point prevalence, number affected in given time/ period.

Question 17

Cross sectional study

- Definition

Answer 17

Measures prevalence and exposure of disease

Question 18

Exposure

Answer 18

Proportion of people who undertake a health behaviour in specific populations.

Question 19

Cohort studies

- Definition

Answer 19

Study where people who presently have a certain condition or receive a particular treatment are followed over time and compared with another group of people who are not affected by the condition.

Measures incidence and risk over a certain period of time.

Question 20

Incidence

• Definition
• Equation

Answer 20

The number of onsets of disease or health event, in given period of a defined population.

Number of new events in population/ Average number of people exposed to risk during this period.

Question 21

Relative risk

Answer 21

The incidence of disease/health event when exposed to a factor / incidence without exposure.

In a given time and population.

Question 22

Risk ratio

Answer 22

Incidence of something with exposure/ incidence without exposure

Question 23

Risk difference

Answer 23

Absolute difference between two risks/ incidence in at a specific time.

Calculated by subtracting risk (incidence) between two population

Question 24

Strengths of cohort study [4]

Answer 24

Measures incidence and compares between exposed and unexposed groups.

Studies multiple effects of single exposure.

When prospective, can choose what variables to measure.

Prevents recall bias (when participants are unable to accurately recall events or experiences)

Question 25

Limitations of cohort study [4]

Answer 25

- Takes time, might lose relevance - Prospective study is expensive - Loss of participants due to follow up - Requires large population.

Question 26

P value

Answer 26

Probability that difference observed could have occurred by chance if the groups compared were alike. Lower p value= less likely to have occurred by chance. Greater sample size= smaller P value, narrower confidence interval.

Question 27

Confidence interval

Answer 27

Range of value with a given probability, that the true value of a variable is contained within. Intervals become narrower as sample size increases. - Required when you need to measure effect or give estimates.

Question 28

Sensitivity

Answer 28

Proportion of subjects with condition who correctly test positive for the condition.

Question 29

Specificity

Answer 29

Proportion of subjects without the condition who correctly test negative for the condition.

Question 30

Positive predictive value

Answer 30

The probability that subjects with a positive test truly have the disease. In 2x2 table= A/ A+B As prevalence increases, PPV increases and NPV decreases

Question 31

Negative predictive value

Answer 31

Probability that subjects with negative screening test do not have the disease. In 2x2 table= D/C+D As prevalence decreases, NPV increases, PPV decreases

Question 32

Benefits of RCT (5 )

Answer 32

Reduces selection bias due to randomisation - Confounders are equally distributed when blinded - Decreases observer/ research bias (measurements won't be different if intervention allocation is known) - Reduces performer bias (different treatment of group if allocation is known) High internal validity

Question 33

Components of sample size calculation

Answer 33

Statistical power - Type 2 error - 80-90% of correctly rejecting the null hypothesis and accepting it, when difference exists. - <20% chance of accepting null hypothesis when there is true difference Type 1 error - <5% of falsely rejecting null hypothesis when there is no difference Smallest effect of interest - Minimally clinically relevant different - Minimal difference between groups that is biologically plausible/ clinically relevant Variance - Variability of outcome measure (SD)