How do you calculate the sensitivity?
TP/(TP+FN)
TP = true positive
FN = false negative
What is the standard error of the mean
Equation to work it out
how ‘accurate’ the calculated sample mean is from the true population mean
Standard deviation/ square root (sample size)
Odds vs probability
Probability of rolling 6 on a dice is 1/6. Odds are 1/5 (odds = ratio of those who incur an outcome vs those who do not incur the outcome) - important for working out odds ratio
Equation for positive predictive value
TP/ (TP + FP)
What is specificity
proportion of patients without the condition who have a negative test result
What is relative risk?
Relative risk (RR) is the ratio of risk in the experimental group (experimental event rate, EER) to risk in the control group (control event rate, CER).
How do you calculate relative risk reduction?
(EER-CER)/CER
EER = rate at which events occur in the experimental group
CER = rate at which events occur in the control group
When is hazard ratio used?
Used when analysing survival over time
Do you include patients who drop out of RCT? What is this type of analysis called?
Yes - intention to treat analysis.
Intention to treat analysis is done to avoid the effects of crossover and drop-out, which may affect the randomization to the treatment groups.
What is the p value
The probability of obtaining a result by chance at least as extreme as the one that was actually observed, assuming that the null hypothesis is true
Properties of the normal distribution
68.3% of values lie within 1 SD of the mean (above and below)
95.4% of values lie within 2 SD of the mean
99.7% of values lie within 3 SD of the mean
Features of case-control study
Observational and retrospective. Patients with a particular condition (cases) are identified and matched with controls. Data is collected on past exposure
What is the power of a study
The probability of correctly rejecting the null hypothesis when it is false/ probability of detecting a statistically significant difference
How do you calculate the likelihood ratio for a positive test reesult
Sensitivity/ (1-specificity)
How do you calculate the absolute risk reduction
ARR = (control event rate) - (experimental event rate) - if outcome is undesirable
ARR = EER - CER if outcome is desirable
What is variance?
Square root of standard deviation
How do you calculate number needed to treat
1/ absolute risk reduction
What is a funnel plot used for?
To demonstrate the existence of publication bias in meta analyses
What is the p value?
Is the probability of obtaining a result by chance at least as extreme as the one that was actually observed, assuming that the null hypothesis is true
What is the outcome measure in case-control study
Odds ratio
Which statistical test is used for parametric data (normally distributed)
Student’s t-test - paired or unpaired*
Pearson’s product-moment coefficient - correlation
Which statistical tests are used for non-parametric data (not normally distrubuted)
Mann-Whitney U
Wilcoxon signed-rank test
Chi-squared
Sprearmans rank
Which statistical test is used for to compare two sets of observations on a single sample, e.g. a ‘before’ and ‘after’ test on the same population following an intervention
Wilcoxon signed-rank test
Equation to work out prevalence
prevalence = incidence * duration of condition
