Evaluating Prognosis Flashcards
What studies are used to estimate prognosis?
Cohort studies
RCTs
X and Y axis to evaluate prognosis
X: Time
Y: proportion surviving i.e. not attaining an ‘event’
What is displayed at each point on a graph?
Cumulative probability of an individual remaining free of event/outcome
What is the name of the curve for evaluating prognosis?
Kaplan-Meier Survival Curves
What does it mean if observations are censored?
Not all subjects will have reached the outcome by that period of time
What does left censored mean?
Observations began after some events have occurred (missed past events)
What does right censored mean?
Observation was terminated before events could be observed (missed future events)
Why can we not calculate the mean survival time from studies?
Due to censoring we do not know the actual survival times beyond the end of the study period
What is the median survival time?
The survival time of 50% of observed sample
How can one compare survival rates of two samples?
Log-rank test
What type of test is log-rank test?
Non-parametric
What can log-rank test not be used for?
To account or adjust for effects of other variables
How does one study multiple variables in survival studies?
Cox regression analysis
What type of test is cox regression analysis?
Semiparametric
Why is cox regression analysis called proportional hazard
It assumes that the effects of multiple variables are constant over a period of time
What does log-rank test inform us?
Whether significant differences are present between two survival curves
Does not measure magnitude
What tells us the size of difference between two survival curves?
Hazard ratio
What can Mantel-Haenszel test be used for in survival studies?
To study effect of two binary independent variables on outcome
Method used if one categorical IV and continuous DV
Kaplan-Meier curve with log-rank test
Method used if >1 categorical IV and continuous DV
Cox’s proportional hazard
Method used if 2 categorical IV and continuous DV
Mantel-Haenszel test
Questions to ask re validity of results in a prognostic study
Was sample representative?
Were patients similar to each other with respect to prognostic factors?
Was follow-up sufficiently complete?
Were outcome criteria objective and unbiased?
How to determine if sample of patients is representative
If they go through a filter, likely they will be different from population of interest
How to test if patients were similar to each other with respect to prognostic factors
If study includes patients at different stages of the disease, survival estimates of the group will not be applicable to each group member
