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Rmd 222d2c4 Dave Tang 2025-12-05 Odds ratio

Introduction

Odds ratios are widely used to measure the strength of association between two variables. Odds and probability are related but they are different:

  • Probability: Chance of something happening out of all possibilities
  • Odds: Ratio of something happening to it NOT happening
sick <- 2
healthy <- 8
total <- sick + healthy

probability <- sick / total
probability
[1] 0.2
odds <- sick / healthy
odds
[1] 0.25

An odds ratio compares the odds of an event between two groups.

Smoking and Lung Cancer

A sample of 200 people broken down into four categories.

data <- matrix(
  c(20, 80, 2, 98),
  nrow = 2,
  byrow = TRUE,
  dimnames = list(
    c("Smokers", "Non-smokers"),
    c("Cancer", "No Cancer")
  )
)

data
            Cancer No Cancer
Smokers         20        80
Non-smokers      2        98

Calculate odds of cancer for the two groups.

odds_smokers <- data[1, 1] / data[1, 2]
odds_nonsmokers <- data[2, 1] / data[2, 2]

odds_smokers
[1] 0.25
odds_nonsmokers
[1] 0.02040816

Calculate odds ratio.

odds_ratio <- odds_smokers / odds_nonsmokers
odds_ratio
[1] 12.25

Smokers have 12.25 times the odds of lung cancer compared to non-smokers.

Using Fisher’s Exact Test

Use fisher.test() for calculating odds ratios with confidence intervals.

fisher.test(data)

    Fisher's Exact Test for Count Data

data:  data
p-value = 5.091e-05
alternative hypothesis: true odds ratio is not equal to 1
95 percent confidence interval:
   2.809877 110.218221
sample estimates:
odds ratio 
  12.12786 

Logistic Regression Example

Odds ratios from logistic regression models.

set.seed(1984)
n <- 200
age <- rnorm(n, mean = 50, sd = 15)
smoker <- rbinom(n, 1, 0.3)
disease <- rbinom(n, 1, plogis(-5 + 0.05*age + 2*smoker))

model <- glm(disease ~ age + smoker, family = binomial)
summary(model)

Call:
glm(formula = disease ~ age + smoker, family = binomial)

Coefficients:
            Estimate Std. Error z value Pr(>|z|)    
(Intercept) -4.53700    0.87994  -5.156 2.52e-07 ***
age          0.04302    0.01493   2.881  0.00396 ** 
smoker       1.83998    0.40800   4.510 6.49e-06 ***
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

(Dispersion parameter for binomial family taken to be 1)

    Null deviance: 188.56  on 199  degrees of freedom
Residual deviance: 157.34  on 197  degrees of freedom
AIC: 163.34

Number of Fisher Scoring iterations: 5

Odds ratios.

or_coef <- exp(coef(model))
or_coef
(Intercept)         age      smoker 
 0.01070543  1.04395510  6.29641114 

sessionInfo()
R version 4.5.0 (2025-04-11)
Platform: x86_64-pc-linux-gnu
Running under: Ubuntu 24.04.3 LTS

Matrix products: default
BLAS:   /usr/lib/x86_64-linux-gnu/openblas-pthread/libblas.so.3 
LAPACK: /usr/lib/x86_64-linux-gnu/openblas-pthread/libopenblasp-r0.3.26.so;  LAPACK version 3.12.0

locale:
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 [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                 
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[11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

time zone: Etc/UTC
tzcode source: system (glibc)

attached base packages:
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other attached packages:
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