Appendix B — Mathematics of logistic regression

In Chapter 11, we introduced logistic regression models for binary response variables. Here we will show some of the mathematics underlying these models, making using of the matrix notation for regression introduced in Appendix A.

B.1 Matrix representation of logistic regression

Given a binary response variable $Y$ and predictors $X_1,X_2,\dots ,X_p$, the logistic regression model is

$$\begin{array}{}\text{(B.1)}& \text{logit}(\pi )=\log (\frac{\pi }{1-\pi })={\beta }_0+{\beta }_1{X}_1+{\beta }_2{X}_2+\cdots +{\beta }_p{X}_p\end{array}$$

where $\pi =Pr(Y=1)$.

Similar to linear regression, we can write a matrix representation of Equation B.1.

$$\begin{array}{}\text{(B.2)}& \text{logit}(\mathit{\pi })=\log (\frac{\mathit{\pi }}{1-\mathit{\pi }})=\mathbf{X}\mathit{\beta }\end{array}$$

We have the following components in Equation B.2:

• $\mathit{\pi }$ is the $n\times 1$ vector of probabilities, such that ${\mathit{\pi }}_i=Pr(y_i=1)$
• $\mathbf{X}$ is the $n\times (p+1)$ design matrix. Similar to linear regression, the first column is $\mathbf{1}$, a column of 1’s corresponding to the intercept.
• $\mathit{\beta }$ is a $(p+1)\times 1$ vector of model coefficients.

Though not directly in Equation B.1 or Equation B.2, the underlying data also includes $\mathbf{y}$, an $n\times 1$ vector of the binary response variables.

We are often interested in the probabilities computed from the logistic regression model. The probabilities computed from Equation B.2 are

$$\begin{array}{}\text{(B.3)}& \mathit{\pi }=\frac{e^{\mathbf{X}\mathit{\beta }}}{1+e^{\mathbf{X}\mathit{\beta }}}\end{array}$$ See Section 11.2 for more detail about the relationship between the logit, odds, and probability.

B.2 Estimation

We want to find estimates $\hat{\mathit{\beta }}$ that are the best fit for the data based on the model in Equation B.2. In Section 11.4, we outlined how we use maximum likelihood estimation to find $\hat{\mathit{\beta }}$. Here we will show more of the mathematical details behind the model estimation.

Let $Z$ be a random variable that takes values 0 or 1. Then $Z$ follows a Bernoulli distribution such that

$$Pr(Z=z)=p^z(1-p{)}^{1-z}$$

where $p=Pr(z=1)$.

$E(Z)=p$ and $Var(Z)=p(1-p)$.

The response variable follows a Bernoulli distribution, such that $P(Y=y_i)={\pi }_i^{y_i}(1-{\pi }_i{)}^{1-{\pi }_i}$. Let ${\mathbf{x}}_i^{\mathsf{T}}$ be the $i^{th}$ row of the design matrix $\mathbf{X}$. Then, using Equation B.3, we have

$$\begin{array}{}\text{(B.4)}& P(Y=y_i)=(\frac{e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }}}{1+e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }}}{)}^{y_i}(1-\frac{e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }}}{1+e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }}}{)}^{1-y_i}\end{array}$$ that the likelihood function is a measure of how likely we observe the data given particular values of the model parameters $\hat{\mathit{\beta }}$.

Let $Z_1,Z_2,\dots ,Z_n$ be independent Bernoulli random variables. The joint distribution of $Z_1,Z_2,\dots ,Z_n$ (the probability of observing these values) is

$$f(Z_1,Z_2,\dots ,Z_n)=\prod _{i=1}^n{p}_i^{z_i}(1-p_i{)}^{1-z_i}$$

where $p_i=Pr(Z_i=1)$ .

Using Equation B.4, the likelihood function for logistic regression is

$$\begin{array}{}\text{(B.5)}& L(\mathit{\beta }|\mathbf{X},\mathbf{y})=\prod _{i=1}^n(\frac{e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }}}{1+e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }}}{)}^{y_i}(1-\frac{e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }}}{1+e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }}}{)}^{1-y_i}\end{array}$$

To make the math more manageable, we will maximize the log likelihood shown in Equation B.6. Maximizing Equation B.6 is equivalent to maximizing Equation B.5.

$$\begin{array}{}\text{(B.6)}& \begin{array}{rl}\log L(\mathit{\beta }|\mathbf{X},\mathbf{y})& =\sum _{i=1}^n{y}_i\log (\frac{e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }}}{1+e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }}})+\sum _{i=1}^n(1-y_i)\log (1-\frac{e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }}}{1+e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }}})\\ & [\text{Given }1-\frac{e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }}}{1+e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }}}=\frac{1}{1+e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }}}]\\ & =\sum _{i=1}^n{y}_i\log (e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }})-\sum _{i=1}^n{y}_i\log (1+e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }})-\sum _{i=1}^n\log (1+e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }})+\sum _{i=1}^n{y}_i\log (1+e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }})\\ & =\sum _{i=1}^n{y}_i{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }-\sum _{i=1}^n\log (1+e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }})\end{array}\end{array}$$

We take the first derivative of Equation B.6 with respect to $\mathit{\beta }$. An outline of the steps is shown below. The maximum likelihood estimator is the vector of coefficients such that is the solution to $\frac{\partial \log L}{\partial \mathit{\beta }}=0$ .

$$\begin{array}{}\text{(B.7)}& \begin{array}{rl}\frac{\partial \log L}{\partial \mathit{\beta }}& =\sum _{i=1}^n{y}_i{x}_i^{\mathsf{T}}\mathit{\beta }-\sum _{i=1}^n\log (1+e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }})\\ & =\sum _{i=1}^n{y}_i{\mathbf{x}}_i^{\mathsf{T}}-\frac{e^{{\mathbf{x}}_i^{\mathbf{T}}\mathit{\beta }}{\mathbf{x}}_i^{\mathsf{T}}}{1+e^{{\mathbf{x}}_i^{\mathsf{T}}\mathit{\beta }}}\end{array}\end{array}$$

There is no closed-from solution for this, i.e., there is no neat formula for $\hat{\mathit{\beta }}$ as we found in Section A.3.1 for linear regression. Therefore, numerical approximation methods such are used to find the maximum likelihood estimators $\hat{\mathit{\beta }}$. One popular method is Newton-Raphson, a “root-finding algorithm which produces successively better approximations to the roots (or zeroes) of a real-valued function” (Wikipedia contributors 2025). Numerical approximation methods such as Newton Raphson systematically search the space of of possible values of $\hat{\mathit{\beta }}$ until it converges on the solution (the “root”) to Equation B.7.

B.3 Inference for logistic regression

In Section 11.5, we introduced inference for a single coefficient ${\beta }_j$ in the logistic regression model. Because there is no closed-form solution for the maximum likelihood estimator $\hat{\mathit{\beta }}$ found in Section B.2, there is no closed form solution for the mean and variance for the distribution of $\hat{\mathit{\beta }}$. We rely on theoretical results to know the distribution of $\hat{\mathit{\beta }}$ as $n$ gets large (called asymptotic results).

Given $n$ is large,

$$\begin{array}{}\text{(B.8)}& \hat{\mathit{\beta }}\sim N(\mathit{\beta },({\mathbf{X}}^{\mathsf{T}}\mathbf{V}\mathbf{X}{)}^{-1})\end{array}$$

where $\mathbf{V}$ is an $n\times n$ diagonal matrix, such that $V_{ii}$ is the estimated variance for the $i^{th}$ observation.¹

The standard error used for hypothesis testing and confidence intervals for a single coefficient ${\beta }_j$ , is computed as the $j^{th}$ diagonal element of $Var(\hat{\mathit{\beta }}{)}^{1/2}=({\mathbf{X}}^{\mathsf{T}}\mathbf{V}\mathbf{X}{)}^{-1/2}$. This is why the hypothesis tests and confidence intervals in Section 11.5.2 are only reliable for large $n$, because they depend on asymptotic approximations in Equation B.8. We can use simulation-based methods if the data has a small sample size.

Wikipedia contributors. 2025. “Newton’s Method.” https://en.wikipedia.org/wiki/Newton%27s_method.

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1. Recall that the variance of the Bernoulli distribution depends on $\pi$, so each observation has a different variance. This is in contrast to linear regression, where all observations have the same variance ${\sigma }_{ϵ}^2$.