Tuesday - Logistics regression analysis

12 important questions on Tuesday - Logistics regression analysis

When can you use 2x2 tables?

When outcome is binary/dichotomous with 2 levels

How do you calculate the odds ratio (OR)?

OR = odds yes / odds no = 0.1215/0.074 = 1.635

But! Total number of people is not needed as it cancels out: OR = 102*2058 / 153*839

Why is it possible to calculate the odds ratio in a case-control study and not the relative risk?

Because the odds ratio does not depend on number of people, because the proportion is enough. The relative risk does need the number of people in the study, but in a case-control study is the number of non-cases chosen by the designers of the experiments
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How do you calculate probability from odds?

P(x) = odds/(1+odds)

Which number is associated with no relation in odds?

1
So, if there is no relationship 1 is in the confidence interval

What is the difference between linear regression and logistic regression?

Logistic regression is about probabilities, which always lie within the range 0-1. Linear regression models can exceed 0 and 1.

What is the formula for logistic models?

See picture
--> not valid in case control study!!!

The logistic model can also be defined in terms of odds, what is this equation?

Odds = P(x) / 1-P(x) = exp(ß0+ß1x)

How can exp(a+b) also be written?

Exp(a)*exp(b)

When is the logistic model not valid?

Case control study

What are the assumptions of logistic models?

- linear relationship between x and log odds
- observations independent
- residuals  random
- x can be measured without error

What is the formula to calculate the 95% CI of ß and of the odds ratio?

CIß = ß +- 1.96*SE(ß)
CI(OR) = exp(CIß)

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