Statistics and Epidemiology Flashcards
What is standard deviation?
Square root of variance
What is variance?
Standard deviation squared
What is the definition of sensitivity?
The proportion of individuals with the disease that were correctly identified by the test
What is the definition of specificity?
The proportion of individuals who were confirmed not to have the disease who were correctly identified as normal by the test
What is a method of estimating distribution?
Histogram
What test is used to compare means between two separate groups of PARAMETRIC data?
Independant student t-test
What test is used to compare means between two separate groups of NON-PARAMETRIC data?
Mann-Whitney U
What test is used to compare results between two paired sample of PARAMETRIC data?
Paired student t-test
What test is used to compare results between two paired sample of NON-PARAMETRIC data?
Wilcoxon signed rank
What test is used to compare three or more measurements on one subject of PARAMETRIC data?
Repeated ANOVA
What test is used to compare three or more measurements on one subject of NON-PARAMETRIC data?
Friedman
What test is used to compare one variable between three or more separate variables of PARAMETRIC data?
One-way ANOVA
What test is used to compare one variable between three or more separate variables of NON-PARAMETRIC data?
Kruskal Wallis
What test is used to look for relationships between two continuous variables of PARAMETRIC data? (i.e. correlation)
Pearson coefficient
(Pearson = parametric)
What test is used to look for relationships between two continuous variables of NON-PARAMETRIC data? (i.e. correlation)
Spearman coefficient
What is level of evidence IV?
Panel of experts - weakest form of evidence
What is level of evidence III?
Evidence from case, correlation, and comparative studies
What is level of evidence IIb?
Evidence from at least one well designed experimental trial
What is level of evidence IIa?
Evidence from at least one well designed controlled trial which is not randomized
What is level of evidence Ib?
Evidence from at least one Randomized Controlled Trial
What is level of evidence Ia?
Evidence from Meta-analysis of Randomized Controlled Trials
How is SEM (standard error of the mean) calculated?
SEM = SD/square root of sample size
What is standard error?
Allows you to calculate how different the sample mean is likely to be from the population mean
It is a proxy for sampling error
Large SE - LESS likely to represent population
Small SE - MORE likely to represent a population
How do you calculate 95% confidence intervals using SEM?
(mean - 1.96SEM) to (mean + 1.96SEM)
What is the absolute risk?
= the actual risk
What is relative risk?
= the ratio of risk comparing the risk in exposed vs. controlled groups
How is sensitivity calculated?
A/(A+C)
How is specificity calculated?
D/(B+D)
How is negative predictive value (NPV) calculated?
D/(C+D)
What is the NPV?
If the result is negative what is the chance that the patient is ACTUALLY negative
How is positive predictive value (PPV) calculated?
A/(A+B)
What is the PPV?
If the result is positive, what is the chance that the patient is ACTUALLY positive
What is the negative likelihood ratio?
(1-sensitivity)/specifity
A multiplier for how much the risk of having the condition is decreased if the test is negative
What is the positive likelihood ratio?
sensitivity/(1-specifity)
A multiplier for how much more the patient is likely to have the condition if the result is positive
What is the most appropriate method to assess prognosis?
Cohort study
What do parametric tests assume?
Assume a normal distribution of population data for the variable being tested and are used for testing variables within a population that are interval or ratio e.g. height, temperature or age
What type of tests are used when the population distribution is not known, or variables are of a nominal or ordinal value e.g. eye colour or marital status?
Non-parametric
What are the parametric tests?
Pearson; t-test; f-test; ANOVA; z-test
What are the non-parametric test?
Spearman; Mann-Whitney; Chi2; Wilcoxen signed rank; Fisher exact probability; Kruskal Wallis; Friedman
What are type I errors?
Incorrect rejection of a null hypothesis
False +ve
Detecting an effect that is not present
What are the type II errors?
Incorrect acceptance of a null hypothesis
False -ve
Failing to detect an effect that is present
What are ROC curves?
y = sensitivity
x = 1-specificty
What is the most appropriate method to assess diagnostic tests?
Cross-sectional studies
What is the most appropriate method to assess treatment/intervention?
RCT
How should the power of a study be described?
The study’s ability to detect an effect of a specified size?
What is the definition of incidence?
Number of new cases of a disease (usually per 100,000) that occur in a population previously free of it
What is the definition of prevalence?
The frequency of existing disease at a given time
E.g. prevalence of ovarian cancer greatest in 60-69 age group, but incidence is greatest in the 80+ group
What is a Kaplan-Meier plot used to graphically represent?
Probability of survival
What are case-control studies useful for?
Rare diseases
What are different types of categorical data?
Nominal/binary
Ordinal
What are different types of quantitive data?
Continuous
Discrete
What is mean sensitive to?
Outliers (extreme values)
The higher the power of a study, the lower the probability of what?
The higher the power of a study, the lower the probability of type II errors
High sensitivity is required for which sort of tests?
Screening tests, thus minimising missed cases
High specificity is required for which sort of tests?
Diagnostic tests, thus minimising misdiagnoses
What is the only thing the magnitude of the number needed to harm (NNH) depends on?
The absolute difference between treatment
groups in risk of the adverse event
What does concealed allocation aim to do in an RCT?
Eliminate selection bias
What are multifactoral RCTs?
When 2+ interventions are compared to the control
What are crossover RCTs?
Where each participant is their own control (i.e. they have 2 different treatments at 2 different times)
What is the principle of ‘intention to treat’ in RCTs?
The two groups of patients were compared on the basis of the treatment regimen assigned at the start of the trial
What is the principle of ‘per protocol analysis’ in RCTs?
Only those fully compliant are included
What is interim analysis in RCTs?
An independant body that monitors the treatment effects and early signs that it is working. Good for large/ethically dubious trials
What are odds?
No. of time an event happens / no. of times an event DOESN’T happen
What is an odds ration?
Odds of exposure to a RF in case group / Odds of exposure to a RF in control group
What is absolute risk reduction (ARR)?
Difference in event rate in the intervention group and control group - i.e. the improvement in the intervention group minus the improvement in control group
What is relative risk reduction?
ARR/placebo no improvement event rate
What is NNT?
100/ARR
What is relative risk?
Relative risk (RR) is the ratio of risk in an exposed group to a non exposed group
RR = Probability of an event when exposed/Probability of event in control group
What is an absolute risk increase?
The absolute difference between treatment groups in the risk of adverse event
What is the NNH?
100/ARI
How do you calculate 95% CI for mean?
(Mean - 1.96xSEM) to (Mean +1.96xSEM)
How do you calculate the upper CI limit?
Mean + (1.96 x SEM)
How do you calculate the lower CI limit?
Mean - (1.96 x SEM)
When would a risk be considered ‘very common’?
1 in 1 to 1 in 10
When would a risk be considered ‘common’?
1 in 10 to 1 in 100
When would a risk be considered ‘uncommon’?
1 in 100 to 1 in 1000
When would a risk be considered ‘rare’?
1 in 1000 to 1 in 10000
When would a risk be considered ‘very rare’?
Less than 1 in 10 000
What is the ideal NNT?
1
What is acquiescence bias?
‘Yes’ bias - when participants tend to agree with you/what you are saying
What is reporting bias?
When researchers selectively report their findings/make omissions
When is median used?
When date is skewed
What does it mean when the CI includes zero?
> 5% chance (with 95% CIs) that there was no true change + the intervention is ineffective
What is a p value?
The probability of any observed difference having happened by chance - i.e. lower the p value the more likely the effect is to be true
What does concealed allocation aim to do?
To eliminate selection bias
What does randomisation aim to do?
Prevent selection bias
What does double-blinding aim to do?
To eliminate measurement bias
What is exclusion bias?
When there are systematic difference in those that have withdrawn from the trial
What is evidence is required to meet an FSRH ‘good practice point’?
Clinical experience of the guideline development group?
Are ascertainment and sampling bias the same thing?
Yes
Are measurement and detection bias the same thing?
Yes
