PAS - Sample Estimates & Confidence Intervals

Question 1

what is a confidence interval?

Answer 1

alternative to p-values

provides a range of values where we can be X% confident the population parameter lies

Question 2

potential biases that can arise from estimates based on population samples? (3)

Answer 2

selection bias = sample isn’t representative of population

measurement bias = errors in measuring outcomes arise - e.g. from using an inaccurate instrument

publication bias = when positive results are more likely to be published than negative or null results, skews evidence base

Question 3

how does a smaller sample size affect the confidence interval?

Answer 3

smaller sample size = wider confidence interval = greater uncertainty with estimate

Question 4

how does a larger sample size affect the confidence interval?

Answer 4

larger sample size= narrower CL = more precise estimates

Question 5

describe CL interpretation around absolute differences

Answer 5

CL around an absolute difference directly shows the range of possible values for the difference in outcomes

e.g. 95% CL of 2-8% for an absolute risk reduction of 5% = 95% confident the true risk reduction is between 2-8%

Question 6

describe CL & risk ratios

Answer 6

CL for a (risk) ratio shows the range of possible values for the ratio of outcomes between groups

e.g. relative risk ratio for developing a disease with a new treatment is 0.75 with a 95% CL between 0.6-0.9 = 95% confident that the true relative risk lies between 0.6-0.9
- treatment reduces risk by 25%

Question 7

calculating & interpreting risk ratio

Answer 7

risk of outcome with treatment divided by risk of outcome with placebo = produces a unitless ratio

RR = 1 - no outcome

RR < 1 - reduction in risk = ratio can be represented as a negative percentage

e.g. 0.91 RR
= -9% proportionate reduction in risk
= -0.09% absolute reduction in risk (per 100 people)

Question 8

key concepts in the study design of clinical trials

Answer 8

randomisation - lessens selection bias, ensures comparable groups

control group

blinding

placebo-controlled group = helps measure true effect of the intervention

ethical considerations

Question 9

point estimates - interpretation?

Answer 9

provides a single best estimate of the effect of treatment

Question 10

confidence intervals - interpretation?

Answer 10

indicate the range of values within which the true effect likely lies

Question 11

probability/ p-values - interpretation?

Answer 11

assess the evidence against the null hypothesis - lesser than the alpha threshold suggests a statistically significant difference

Question 12

compare 95% reference range vs 95% confidence interval?

Answer 12

(95%) reference range = measure of the spread of the continuous numerical data only
- mean +/- 2 standard deviations

(95%) confidence interval = measure of the precision of a sample estimate (95% chance that the interval contains the true population value for a certain parameter)
- mean +/- 2 standard errors

Question 13

standard deviation vs standard error?

Answer 13

SD = variability within a single sample, calculated from sample data

SE = variability across multiple samples of a population, estimated value