Critical Appraisal 3 Flashcards
What is meant by a cohort study?
Cohort
take a group of patient and follow them for a period of time.
Split them by RISK FACTOR
looking to see what impact this has on a set out outcomes.
Gives relative risks.
Can be prospective ie. split/define groups BEFORE data is collected
alternative is retrospective, where split/define happens AFTER data is collected but still looks for cause and effect
proceeds from cause of a disease, to its effects
What is meant by a case control study?
Case-control
take a group of people with one particular OUTCOME OF INTEREST, and a group
without this outcome.
Look back retrospectively to see if can ascertain factors which are more
common in one group than the other.
Can only ever prove CORRELATION, outcomes defined as odds ratios
Proceeds from the effects of a disease to its cause
What are the differences between a cohort study and case control study?
Cohort
- usually prospective (can be retrospective)
- gives relative risk - can prove some causation
Case-control
- retrospective look back in patients with one outcome of interest e.g lung cancer, versus group without lung cancer
- can only ever prove correlation using an odds ratio - e.g smoking is strongly related with lung cancer
What is meant by a cross-sectional study?
data from a single time point across a population; a point prevalence study, for
instance.
Can see correlations, generate hypotheses, cannot prove cause and effect.
What is meant by a longitudinal study?
multiple data points over time, often over years.
Tract individuals through time.
Cohort studies come under this.
What are benefits of case control over cohort?
What are benefit of cohort over case control?
Case control -
smaller sample size
does not take long, as you already have patients with outcome
ideal for rare conditions
Cohort -
requires a larger sample size
take a long time
not good for rare conditions as need large sample size
you get an actual estimate of how much a risk can cause a disease
can determine incidence
What is difference between case-control study and retrospective cohort study?
Case-control
- starts with cases (lung cancer) and controls (healthy), looks back retrospectively gathering new data to help see if there was a cause found for cases e.g smoking
Retrospective cohort
- starts with cohort selected on exposure status e.g exposed smoking v not exposed smoking (not cases/ controls)
- limits what data has been collected .eg inaccurate/ missing data. But much cheaper/ quicker than case-control
Lung cancer v healthy - look back for cause. This is a case control study. Generate odds ratio
Smoker v non-smoker - look back to see who develops cancer.
This is a cohort study. Generate relative risk
What is test sensitivity?
proportion of those who have disease, who test positive
true positive/ true positive + false negatives
What is test specificity?
proportion of those who do not have disease, who test negative
true negative/ true negative + false positive
What is PPV?
likelihood that a positive test result, means person has disease
true positive/ true positive + false positive
What is NPV?
likelihood that a negative result, means person does not have disease
true negative/ true negative + false negative
What is absolute risk?
Number of people experiencing an event, in relation to total population, over a period of time
e.g 1 out of 100 people get cancer in a population in a year
Therefore absolute risk is 1%
What is relative risk?
Comparison between two groups of people
e.g 1 out of 100 people get cancer in a population. 5 out of 100 people get cancer in another population
There is a 20% increase risk of cancer in second population. Although the absolute risk is 5%
Say men have a 2 in 20 risk of developing a certain disease by the time they reach the age of 60. Then, say research shows that a new treatment reduces the relative risk of getting this disease by 50%. The 50% is the relative risk reduction, and is referring to the effect on the 2. 50% of 2 is 1. So this means that the absolute risk is reduced from from 2 in 20, to 1 in 20.
What is number needed to treat?
NNT - number of patients you need to treat to prevent one additional bad outcome
NNT = 1/ absolute risk reduction
mortality 80% without it
40% with it = 40% absolute risk reduction therefore NNT = 1/0.4 = 2.5
if morality 80% without it
70% with it = 10% absolute risk reduction therefore NNT = 1/0.1 = 10
if mortality 80% without it
75% with it = 5% absolute risk reduction therefore NNT = 1/0.5 = 20
mortality 80% without it
40% with it = 40% absolute risk reduction therefore NNT = 1/0.4 = 2.5
note this example has a 50% relative risk reduction. But in NNT we use absolute risk
