# Notation¶

This book covers topics from multiple disciplines. Unfortunately, some of these disciplines use the same notation to describe different concepts. For clarity, we have devised notation that may differ slightly from the notation used in your discipline.

A population parameter is denoted by $$\theta^*$$. The model parameter that minimizes a specified loss function is denoted by $$\hat{\theta}$$. Typically, we desire $$\hat{\theta} \approx \theta^*$$. We use the plain variable $$\theta$$ to denote a model parameter that does not minimize a particular loss function. For example, we may arbitrarily set $$\theta = 16$$ in order to calculate a model’s loss at that choice of $$\theta$$. When using gradient descent to minimize a loss function, we use $$\theta^{(t)}$$ to represent the intermediate values of $$\theta$$.

We will always use bold lowercase letters for vectors. For example, we represent a vector of population parameters using $$\boldsymbol{\theta^*} = [ \theta^*_1, \theta^*_2, \ldots, \theta^*_n ]$$ and a vector of fitted model parameters as $$\boldsymbol{\hat{\theta}} = [\hat{\theta_1}, \hat{\theta_2}, \ldots, \hat{\theta_n} ]$$.

We will always use bold uppercase letters for matrices. For example, we commonly represent a data matrix using $$\boldsymbol X$$.

We will always use non-bolded uppercase letters for random variables, such as $$X$$ or $$Y$$.

When discussing the bootstrap, we use $$\theta^*$$ to denote the population parameter, $$\hat{\theta}$$ to denote the sample test statistic, and $$\tilde{\theta}$$ to denote a bootstrapped test statistic.