Tools of the trade Flashcards
what is null hypothesis
no significant difference`
what is P value
If p<0.05 statistically significant (reject null hypothesis), if p>0.05 no significant difference.
(95%) Confidence interval
an estimated range of values calculated from a given set of sample data which are likely to contain the ‘true’ population value. If value falls between the range can say “95% sure the result is not due to chance”
what is Attributable Risk
Incidence in the exposed – Incidence in the unexposed/100
what is relative risk
Incidence in the exposed/ Incidence in the unexposed
what is the incidence in exposed
A/(A+B)
A is cases exposed, B is non- cases exposed
what is the incidence in unexposed
C/(C+D)
C is cases non exposed, D is non-cases exposed
why is the attributable risk useful
its useful in evaluating the impact of introduction or removal of risk factors. Its value indicates the number of cases of the disease among the exposed group that could be prevented if the exposure were completely eliminated.
what is the relative risk used for
used as a measure of association between an exposure and disease.
what does it mean if relative risk is less than 1
risk of outcome among exposed is less than risk of outcome among the not exposed. the exposure is associated with a lower risk of the outcome.
what does it mean if relative risk equals 1
indicates that the incidence of disease in the exposed and the unexposed are identical and thus the data shows no association between the exposure and the disease.
what does it mean if relative risk is more than 1
risk of outcome among the exposed is greater than the risk of outcome among the not exposed. the exposure is associated with a higher risk of the outcome
define odds ratio
odds of exposure among cases/odds of exposure among controls.
Odds ratio =
it equals (AxD) / (BxC). A is exposed cases, B is exposed controls, C is non exposed cases and D is non exposed controls
what does it mean if odds ratio = 1
no association
what does it mean if odds ratio is greater than 1
positive association between exposure & disease. The exposure is associated with increased risk of disease
what does it mean if odds ratio is less than 1
inverse association b/t exposure and disease. The exposure is associated with reduced risk of disease.
the lower the P value
the less likely that the estimated difference is a chance finding
what is a case
an individual with the outcome under study (in a casecontrol study
what is a case control study
study in which individuals are selected on the basis of whether or not they have the outcome of interest; usually some relatively rare outcome. Exposure status is explored to establish whether the exposure is more common in the case or control group. This type of study always results in an odds rati
what is count
the most basic measure of disease frequency is a simple count of affected individuals. The number (count) of cases that occurred in a particular population is of little use in comparing populations and groups.
what is a chi squared test
– a statistical procedure for testing whether two proportions are similar
what is a cohort study
tudy in which individuals are selected on the basis of exposure status and are followed over a period of time to allow the frequency of occurrence of the outcome of interest in the exposed and non exposed groups to be compared
what can we say for a 95% confidence interval of 1.2 – 3.4
hat we are 95% confident that the true value of risk will not be lower than 1.2 and will not be higher than 3.4.
what is a confounding variable
a factor that is associated with both the exposure and outcome of interest
what does exposure mean
when people have been ‘exposed’, they have been in contact with something that is hypothesised to have an effect on health
Population attributable risk (also known as the population excess risk)
a measure of the risk of outcome in the study population which is attributable to the exposure of interest.
Population excess fraction (also known as the population attributable fraction)
a measure of the proportion (fraction) of the cases observed in the study population attributable to the exposure of interest.
Regression
a method for controlling the effect of confounding at the analysis stage of a study - statistical modelling is used to control for one or many confounding variables
Stratification
a method for controlling the effect of confounding at the analysis stage of a study - risks are calculated separately for each category of confounding variable, e.g. each age group and each sex separately.
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Standardisation
a method for controlling the effect of confounding at the analysis stage of a study.
Restriction
a method for controlling the effect of confounding at the design stage of a study, e.g. by including
60 patients in a clinical trial only between the ages of 18 and 65 without pre-existing illness so that the results of the trial are not confused (‘confounded’) by different levels of age or morbidity in the two treatment groups.
matching
a method for “controlling for” the effect of confounding at the design stage of a case-control study; controls are selected to have a similar distribution of potentially confounding variables to the cases
