risk and odds ratios and survival analyses - guest video

Question 1

what is risk

Answer 1

the number of participants with outcomes divided by the number of participants

Question 2

what is odds

Answer 2

the number of participants with the outcomes divided by the number of non-outcomes

Question 3

what is the same and whats different : risk vs odds

Answer 3

the numerator is the same : number of ppl with the outcome but the denominator is the different

Question 4

how to calculate risk ratio ?

Answer 4

find the risk of the exposed and the risk of the unexposed
then to find the Relative risk divided the exposed by the unexposed
relative risk is measured by - eg: the relative risk of lunger cancer is 5 times higher in smokers RR= 5

Question 5

when do we use odds instead of risk

Answer 5

when we don’t have the denominator - number at risk only people who got the disease
- we compare the odds of smokers vs odds of control then divide those

Question 6

in a cohort study what ratio do we compute?

Answer 6

risk or an odds

Question 7

in a case-control we can only compute what ratio

Answer 7

odds

Question 8

pros and cons of risk ratio

Answer 8

• easier to interpret but less desirable statistical properties

Question 9

risk vs odds ratio

Answer 9

1. when the outcome is rare then the odds and risk ratios are similar
2. when the outcome is common the odds ratio will be much higher than the risk ratio
3. usually an outcome of 15% in exposed and unexposed is “low” - odds and risk are similar

Question 10

what is a differential follow up?

Answer 10

all participants are followed up for the full 10 years - generally does not occur ppl drop out

Question 11

what is censoring?

Answer 11

drop out before the end of a cohort study
- called right censoring

Question 12

what does censoring require?

Answer 12

us to adjust our odds or risk ratios to account for the differences in follow-up times
- if we don’t adjust it creates a bias in our results (especially true if there is a difference in the censoring of exposed and nonexposed)

Question 13

what is left censoring?

Answer 13

when people experience the outcome before entering the cohort study - not usually a problem

Question 14

how to adjust for censoring

Answer 14

instead of using participants, we use the person-years to calculate - find the incidence rate
make an incidence rate ratio
- properly accounts for differential followup

Question 15

the easiest way to account for differential follow-ups

Answer 15

finding incidence rates and finding the incidence rate ratios
- relative risk is not applicable

Question 16

what are Kaplan Meir curves

Answer 16

when we are studying a binary event with variable follow-up time we have two primary pieces of information
1. whether the event occurred - yes/no
2. the time to when the event or the last follow-up occurred

Question 17

Why are Kaplan Meir curve graphs important

Answer 17

1. summarize incidence rate ratios
2. visualize the distribution of survival times with a KM curve
3. show the probability of having no event at each time point

Question 18

What does the Kaplon Meir estimator do?

Answer 18

takes into account the patients who drop out of the study

Question 19

how do you find the probability of survival - Kaplan meir estimate

Answer 19

1-probability of death
eg: 1/95 ppl died = 0.01 therefore 1-0.01 = 0.98 = the survival rate of that year

Question 20

calculating survival probability

Answer 20

read off the survival curve
find the year and then find the corresponding probability
curve gives us the info

Question 21

What and when is the media survival time calculated/

Answer 21

• used as a summary statitsic
  this is the point at which 50% of patients have died on KM curve
  half of the participants died before this point and half will die after

Question 22

what happens when follow up times differ in a cohort

Answer 22

any summary of the data must account for this