Statistics and studies Flashcards
What is selection bias?
Error in assigning individuals to groups leading to differences which may influence the outcome.
What is recall bias?
Difference in the accuracy of the recollections retrieved by study participants, possibly due to whether they have disorder or not.
This is a problem in case control studies
What is publication bias?
Failure to publish results from valid studies, often as they showed a negative or uninteresting result. Important in meta-analyses where studies showing negative results may be excluded.
What is work up bias?
In studies which compare new diagnostic tests with gold standard tests, work-up bias can be an issue. Sometimes clinicians may be reluctant to order the gold standard test unless the new test is positive, as the gold standard test may be invasive (e.g. tissue biopsy).
What is expectation bias?
Only a problem in non-blinded trials. Observers may subconsciously measure or report data in a way that favours the expected study outcome.
What is the Hawthorne effect?
Describes a group changing it’s behaviour due to the knowledge that it is being studied
What is late look bias?
Gathering information at an inappropriate time e.g. studying a fatal disease many years later when some of the patients may have died already
What is procedure bias?
Occurs when subjects in different groups receive different treatment
What is lead time bias?
Occurs when two tests for a disease are compared, the new test diagnoses the disease earlier, but there is no effect on the outcome of the disease
What are the phases of clinical trial?
Phase 1- Determines pharmacokinetics and pharmacodynamics and side-effects prior to larger studies (first in human - normally healthy people - but not in cancer)
Phase 2 Assess efficacy + dosage
- 2a: Dosing
- 2b: Efficacy
Phase 3 - Assess effectiveness - normally an RTC
Phase 4 - post marketing surveillance. Monitors for long-term effectiveness and side-effects
What do confidence intervals show?
a range of values within which the true effect of intervention is likely to lie
What does the standard error of the mean show?
measure of the spread expected for the mean of the observations - i.e. how ‘accurate’ the calculated sample mean is from the true population mean
How is the standard error of the mean calculated?
SEM = SD / square root (n)
where SD = standard deviation and n = sample size
therefore the SEM gets smaller as the sample size (n) increases
How is the lower 95% confidence interval calculated?
lower limit = mean - (1.96 * SEM)
*If the sample is < 100 then need a student critical T test
How is the higher 95% confidence interval calculated?
upper limit = mean + (1.96 * SEM)
*If the sample is < 100 then need a student critical T test
What is a confounder?
In statistics confounding refers to a variable which correlates with other variables within a study leading to spurious results.
What is correlation?
Correlation is used to test for association between variables
What is regression?
Once correlation is demonstrated can do regression
Regression can be used to predict values of other dependent variables from independent variables.
What is nominal data?
Observed values can be put into set categories which have no particular order or hierarchy.
e.g. Birthplace
What is ordinal data?
Observed values can be put into set categories which themselves can be ordered (for example NYHA classification of heart failure symptoms)
What is discrete data?
Observed values are confined to a certain values, usually a finite number of whole numbers (for example the number of asthma exacerbations in a year)
What is continuous data?
Data can take any value with certain range (for example weight)
What is binomial data?
Data may take one of two values (for example gender)
What is a hazard’s ratio?
The hazard ratio (HR) is similar to relative risk but is used when risk is not constant to time. It is typically used when analysing survival over time
What is incidence?
The incidence is the number of new cases per population in a given time period.
What is the prevalence?
The prevalence is the total number of cases per population at a particular point in time. It can be divided into two types:
- point prevalence = number of cases in a defined population / number of people in a defined population at the same time
- period prevalence = number of identified cases during a specified period of time / total number of people in that population
What is intention to treat analysis?
All patient asigned to one arm of an RCT are analysed in that arm
Regardless of completing assigned treatment of not
What is % is in one standard deviation of a normal distribution?
68.3% of values lie within 1 SD of the mean
What is % is in two standard deviation of a normal distribution?
95.4% of values lie within 2 SD of the mean
What is % is in three standard deviation of a normal distribution?
99.7% of values lie within 3 SD of the mean
What is the numbers needed to treat principal?
Average number of patients who require to be treated for one to benefit compared with a control in a clinical trial.
How is the number needed to treat principal calculated?
1/(Absolute risk reduction) and is rounded to the next highest whole number