Stationarity & introductory functions

$\text{E}([x - \mu]^2) \equiv$ expected deviations of $x$ about $\mu$

$\text{E}([x - \mu]^2) \equiv$ variance of $x = \sigma^2$

We can estimate $\sigma^2$ from a sample as

$$s^2 = \frac{1}{N-1}\sum_{i=1}^N{(x_i - m)^2}$$

If we have two variables, $x$ and $y$, we can generalize variance

$$\sigma^2 = \text{E}([x_i - \mu][x_i - \mu])$$

into covariance

$$\gamma_{x,y} = \text{E}([x_i - \mu_x][y_i - \mu_y])$$

If we have two variables, $x$ and $y$, we can generalize variance

$$\sigma^2 = \text{E}([x_i - \mu][x_i - \mu])$$

into covariance

$$\gamma_{x,y} = \text{E}([x_i - \mu_x][y_i - \mu_y])$$

We can estimate $\gamma_{x,y}$ from a sample as

$$\text{Cov}(x,y) = \frac{1}{N-1}\sum_{i=1}^N{(x_i - m_x)(y_i - m_y)}$$

Correlation is a dimensionless measure of the linear association between 2 variables, $x$ & $y$

It is simply the covariance standardized by the standard deviations

$$\rho_{x,y} = \frac{\gamma_{x,y}}{\sigma_x \sigma_y}$$

$$-1 < \rho_{x,y} < 1$$

Correlation is a dimensionless measure of the linear association between 2 variables $x$ & $y$

It is simply the covariance standardized by the standard deviations

$$\rho_{x,y} = \frac{\gamma_{x,y}}{\sigma_x \sigma_y}$$

We can estimate $\rho_{x,y}$ from a sample as

$$\text{Cor}(x,y) = \frac{\text{Cov}(x,y)}{s_x s_y}$$

Consider a single value, $x_t$

Consider a single value, $x_t$

$\text{E}(x_t)$ is taken across an ensemble of all possible time series

If $\text{E}(x_t)$ is constant across time, we say the time series is stationary in the mean

Stationarity is a convenient assumption that allows us to describe the statistical properties of a time series.

In general, a time series is said to be stationary if there is

1. no systematic change in the mean or variance
   
2. no systematic trend
   
3. no periodic variations or seasonality

Our eyes are really bad at identifying stationarity, so we will learn some tools to help us

For stationary ts, we define the autocovariance function ($\gamma_k$) as

$$\gamma_k = \text{E}([x_t - \mu][x_{t+k} - \mu])$$

which means that

$$\gamma_0 = \text{E}([x_t - \mu][x_{t} - \mu]) = \sigma^2$$

For stationary ts, we define the autocovariance function ($\gamma_k$) as

$$\gamma_k = \text{E}([x_t - \mu][x_{t+k} - \mu])$$

“Smooth” time series have large ACVF for large $k$

“Choppy” time series have ACVF near 0 for small $k$

For stationary ts, we define the autocovariance function ($\gamma_k$) as

$$\gamma_k = \text{E}([x_t - \mu][x_{t+k} - \mu])$$

We can estimate $\gamma_k$ from a sample as

$$c_k = \frac{1}{N}\sum_{t=1}^{N-k}{(x_t - m)(x_{t+k} - m)}$$

The autocorrelation function (ACF) is simply the ACVF normalized by the variance

$$\rho_k = \frac{\gamma_k}{\sigma^2} = \frac{\gamma_k}{\gamma_0}$$

The ACF measures the correlation of a time series against a time-shifted version of itself

The autocorrelation function (ACF) is simply the ACVF normalized by the variance

$$\rho_k = \frac{\gamma_k}{\sigma^2} = \frac{\gamma_k}{\gamma_0}$$

The ACF measures the correlation of a time series against a time-shifted version of itself

We can estimate ACF from a sample as

$$r_k = \frac{c_k}{c_0}$$

The ACF has several important properties:

• $-1 \leq r_k \leq 1$

• $r_k = r_{-k}$

• $r_k$ of a periodic function is itself periodic

• $r_k$ for the sum of 2 independent variables is the sum of $r_k$ for each of them

acf(ts_object)

Recall the transitive property, whereby

If $A = B$ and $B = C$, then $A = C$

Recall the transitive property, whereby

If $A = B$ and $B = C$, then $A = C$

which suggests that

If $x \propto y$ and $y \propto z$, then $x \propto z$

Recall the transitive property, whereby

If $A = B$ and $B = C$, then $A = C$

which suggests that

If $x \propto y$ and $y \propto z$, then $x \propto z$

and thus

If $x_t \propto x_{t+1}$ and $x_{t+1} \propto x_{t+2}$, then $x_t \propto x_{t+2}$

The partial autocorrelation function ($\phi_k$) measures the correlation between a series $x_t$ and $x_{t+k}$ with the linear dependence of $\{x_{t-1},x_{t-2},\dots,x_{t-k-1}\}$ removed

The partial autocorrelation function ($\phi_k$) measures the correlation between a series $x_t$ and $x_{t+k}$ with the linear dependence of $\{x_{t-1},x_{t-2},\dots,x_{t-k-1}\}$ removed

We can estimate $\phi_k$ from a sample as

$$\phi_k = \begin{cases} \text{Cor}(x_1,x_0) = \rho_1 & \text{if } k = 1 \\ \text{Cor}(x_k-x_k^{k-1}, x_0-x_0^{k-1}) & \text{if } k \geq 2 \end{cases}$$

$$x_k^{k-1} = \beta_1 x_{k-1} + \beta_2 x_{k-2} + \dots + \beta_{k-1} x_1$$

$$x_0^{k-1} = \beta_1 x_1 + \beta_2 x_2 + \dots + \beta_{k-1} x_{k-1}$$

We will see that the ACF & PACF are very useful for identifying the orders of ARMA models

Often we want to look for relationships between 2 different time series

We can extend the notion of covariance to cross-covariance

Often we want to look for relationships between 2 different time series

We can extend the notion of covariance to cross-covariance

We can estimate the CCVF $(g^{x,y}_k)$ from a sample as

$$g^{x,y}_k = \frac{1}{N}\sum_{t=1}^{N-k}{(x_t - m_x)(y_{t+k} - m_y)}$$

The cross-correlation function is the CCVF normalized by the standard deviations of x & y

$$r^{x,y}_k = \frac{g^{x,y}_k}{s_x s_y}$$

Just as with other measures of correlation

$$-1 \leq r^{x,y}_k \leq 1$$

ccf(x, y)

Note: the lag k value returned by ccf(x, y) is the correlation between x[t+k] and y[t]

In an explanatory context, we often think of $y = f(x)$, so it’s helpful to use ccf(y, x) and only consider positive lags

A time series $\{w_t\}$ is discrete white noise if its values are

1. independent

2. identically distributed with a mean of zero

A time series $\{w_t\}$ is discrete white noise if its values are

1. independent

2. identically distributed with a mean of zero

Note that distributional form for $\{w_t\}$ is flexible

We often assume so-called Gaussian white noise, whereby

$$w_t \sim \text{N}(0,\sigma^2)$$

We often assume so-called Gaussian white noise, whereby

$$w_t \sim \text{N}(0,\sigma^2)$$

and the following apply as well

    autocovariance:  $\gamma_k = \begin{cases} \sigma^2 & \text{if } k = 0 \\ 0 & \text{if } k \geq 1 \end{cases}$

    autocorrelation:   $\rho_k = \begin{cases} 1 & \text{if } k = 0 \\ 0 & \text{if } k \geq 1 \end{cases}$

A time series $\{x_t\}$ is a random walk if

1. $x_t = x_{t-1} + w_t$

2. $w_t$ is white noise

The following apply to random walks

    mean:   $\mu_x = 0$

    autocovariance:   $\gamma_k(t) = t \sigma^2$

    autocorrelation:   $\rho_k(t) = \frac{t \sigma^2}{\sqrt{t \sigma^2(t + k) \sigma^2}}$

The following apply to random walks

    mean:   $\mu_x = 0$

    autocovariance:   $\gamma_k(t) = t \sigma^2$

    autocorrelation:   $\rho_k(t) = \frac{t \sigma^2}{\sqrt{t \sigma^2(t + k) \sigma^2}}$

Note: Random walks are not stationary

The backshift shift operator ($\mathbf{B}$) is an important function in time series analysis, which we define as

$$\mathbf{B} x_t = x_{t-1}$$

or more generally as

$$\mathbf{B}^k x_t = x_{t-k}$$

For example, a random walk with

$$x_t = x_{t-1} + w_t$$

can be written as

$$\begin{align} x_t &= \mathbf{B} x_t + w_t \\ x_t - \mathbf{B} x_t &= w_t \\ (1 - \mathbf{B}) x_t &= w_t \\ x_t &= (1 - \mathbf{B})^{-1} w_t \end{align}$$

The difference operator ($\nabla$) is another important function in time series analysis, which we define as

$$\nabla x_t = x_t - x_{t-1}$$

The difference operator ($\nabla$) is another important function in time series analysis, which we define as

$$\nabla x_t = x_t - x_{t-1}$$

For example, first-differencing a random walk yields white noise

$$\begin{align} \nabla x_t &= x_{t-1} + w_t \\ x_t - x_{t-1} &= x_{t-1} + w_t - x_{t-1}\\ x_t - x_{t-1} &= w_t\\ \end{align}$$

The difference operator and the backshift operator are related

$$\nabla^k = (1 - \mathbf{B})^k$$

The difference operator and the backshift operator are related

$$\nabla^k = (1 - \mathbf{B})^k$$

For example

$$\begin{align} \nabla x_t &= (1 - \mathbf{B})x_t \\ x_t - x_{t-1} &= x_t - \mathbf{B} x_t \\ x_t - x_{t-1} &= x_t - x_{t-1} \end{align}$$

Differencing is a simple means for removing a trend

The 1st-difference removes a linear trend

A 2nd-difference will remove a quadratic trend

Differencing is a simple means for removing a seasonal effect

Using a 1st-difference with $k = period$ removes both trend & seasonal effects

We can use diff() to easily compute differences

diff(x,
     lag,
     differences
     )

diff(x,
     lag,
     differences
     )

lag $(h)$ specifies $t - h$

lag = 1 (default) is for non-seasonal data

lag = 4 would work for quarterly data or

lag = 12 for monthly data

diff(x,
     lag,
     differences
     )

differences is the number of differencing operations

differences = 1 (default) is for a linear trend

differences = 2 is for a quadratic trend

Characteristics of time series

• Expectation, mean & variance
• Covariance & correlation
• Stationarity
• Autocovariance & autocorrelation
• Correlograms

White noise

Random walks

Backshift & difference operators