Logistic regression is a particular case of the generalized linear model, used to model dichotomous outcomes (probit and complementary log-log models are closely related).

The name comes from the link function used, the logit or log-odds function. The inverse function of the logit is called the logistic function and is given by:

This function takes a value between ]-Inf;+Inf[ and returns a value between 0 and 1; i.e the logistic function takes a linear predictor and returns a probability.

Logistic regression can be performed using the glm function with the option family = binomial (shortcut for family = binomial(link="logit"); the logit being the default link function for the binomial family).

In this example, we try to predict the fate of the passengers aboard the RMS Titanic.

Read the data:

url <- "http://biostat.mc.vanderbilt.edu/wiki/pub/Main/DataSets/titanic.txt"
titanic <- read.csv(file = url, stringsAsFactors = FALSE)

Clean the missing values:

In that case, we replace the missing values by an approximation, the average.

titanic$age[is.na(titanic$age)] <- mean(titanic$age, na.rm = TRUE) 

Train the model:

titanic.train <- glm(survived ~ pclass + sex + age,
                         family = binomial, data = titanic)

Summary of the model:

summary(titanic.train)

The output:

Call:
glm(formula = survived ~ pclass + sex + age, family = binomial, data = titanic)

Deviance Residuals: 
    Min       1Q   Median       3Q      Max  
-2.6452  -0.6641  -0.3679   0.6123   2.5615  

Coefficients:
             Estimate Std. Error z value Pr(>|z|)    
(Intercept)  3.552261   0.342188  10.381  < 2e-16 ***
pclass2nd   -1.170777   0.211559  -5.534 3.13e-08 ***
pclass3rd   -2.430672   0.195157 -12.455  < 2e-16 ***
sexmale     -2.463377   0.154587 -15.935  < 2e-16 ***
age         -0.042235   0.007415  -5.696 1.23e-08 ***
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

(Dispersion parameter for binomial family taken to be 1)

    Null deviance: 1686.8  on 1312  degrees of freedom
Residual deviance: 1165.7  on 1308  degrees of freedom
AIC: 1175.7

Number of Fisher Scoring iterations: 5

• The first thing displayed is the call. It is a reminder of the model and the options specified.

• Next we see the deviance residuals, which are a measure of model fit. This part of output shows the distribution of the deviance residuals for individual cases used in the model.

• The next part of the output shows the coefficients, their standard errors, the z-statistic (sometimes called a Wald z-statistic), and the associated p-values.

  • The qualitative variables are "dummified". A modality is considered as the reference. The reference modality can be change with I in the formula.
  • All four predictors are statistically significant at a 0.1 % level.
  • The logistic regression coefficients give the change in the log odds of the outcome for a one unit increase in the predictor variable.
  • To see the odds ratio (multiplicative change in the odds of survival per unit increase in a predictor variable), exponentiate the parameter.
  • To see the confidence interval (CI) of the parameter, use confint.

• Below the table of coefficients are fit indices, including the null and deviance residuals and the Akaike Information Criterion (AIC), which can be used for comparing model performance.

  • When comparing models fitted by maximum likelihood to the same data, the smaller the AIC, the better the fit.
  • One measure of model fit is the significance of the overall model. This test asks whether the model with predictors fits significantly better than a model with just an intercept (i.e., a null model).

Example of odds ratios:

exp(coef(titanic.train)[3])

 pclass3rd 
0.08797765 

With this model, compared to the first class, the 3rd class passengers have about a tenth of the odds of survival.

Example of confidence interval for the parameters:

confint(titanic.train)

Waiting for profiling to be done...
                  2.5 %      97.5 %
(Intercept)  2.89486872  4.23734280
pclass2nd   -1.58986065 -0.75987230
pclass3rd   -2.81987935 -2.05419500
sexmale     -2.77180962 -2.16528316
age         -0.05695894 -0.02786211

Exemple of calculating the significance of the overall model:

The test statistic is distributed chi-squared with degrees of freedom equal to the differences in degrees of freedom between the current and the null model (i.e., the number of predictor variables in the model).

with(titanic.train, pchisq(null.deviance - deviance, df.null - df.residual
, lower.tail = FALSE))
[1] 1.892539e-111

The p-value is near 0, showing a strongly significant model.