`simts`

VignettesIn this section, we briefly list, describe, and provide the syntax used to simulate time series data using the `simts`

package. The following list includes some basic models available in this package:

- White Noise
`WN()`

- Quantization Noise
`QN()`

- Random Walk
`RW()`

- Drift
`DR()`

- First-order Autoregressive
`AR1()`

- Autoregressive
`AR()`

- First-order Moving Average
`MA1()`

- Moving Average Process
`MA()`

- Gauss-Markov
`GM()`

- Autoregressive Moving Average
`ARMA()`

- Integrated Autoregressive Moving Average
`ARIMA()`

- Seasonal Autoregressive Integrated Moving Average
`SARIMA()`

- Seasonal Autoregressive Moving Average
`SARMA()`

- Sinusoidal process
`SIN()`

- Quantization Noise
`QN()`

Quantization noise is a less known process that is used in engineering applications. It can be described in layperson terms as being a good estimator of a rounding error.

The code below shows how to call the function `gen_gts()`

, which allows the user to generate samples from the above model specifications.

```
# Set seed for reproducibility
set.seed(1337)
# Number of observations
= 10^4
n
# Generate a White Noise Process
= gen_gts(n, WN(sigma2 = 1))
wn
# Generate a Quantization Noise
= gen_gts(n, QN(q2 = .5))
qn
# Generate a Random Walk
= gen_gts(n, RW(gamma2 = .75)) rw
```

By applying the `plot()`

function on the result of a `gen_gts()`

simulation, we can observe a visualization of our simulated data.

```
par(mfrow = c(3,1))
plot(wn)
plot(qn)
plot(rw)
```

Another example with a SARIMA model is given below:

```
# Generate an SARIMA(1,0,1)x(2,1,1)[12]
= gen_gts(n, SARIMA(ar = 0.3, i = 0, ma = -0.27,
sarima sar = c(-0.12, -0.2), si = 1, sma = -0.9,
sigma2 = 1.5, s = 12))
# Plot simulation of SARIMA(1,0,1)x(2,1,1)[12]
plot(sarima)
```

The `simts`

package therefore allows users to easily simulate from a wide variety of classical time series models, but does not limit itself to these models. Indeed, under some restrictions, these models can be combined in different ways to deliver many state-space (latent) models which can be represented as the sum of basic models.

`simts`

’s user friendly interface allows for easy construction of such linear state-space models. In fact, to specify that a certain model is a combination of different models, all that is needed is the “+” symbol between them. For example, consider the following state-space model:

\[\begin{aligned} X_t &= X_{t-1} + \omega + U_t, \;\;\;\;\; U_t \sim \mathcal{N}(0,\gamma^2),\\ Y_t &= X_t + Z_t , \;\;\;\;\; Z_t \sim \mathcal{N}(0,\sigma^2), \end{aligned}\]

it is easy to see that this model is exactely equivalent to the sum of a random walk (with inivation variance \(\gamma^2\)), a linear drift (with slope \(\omega\)) and a white noise process (with variance \(\sigma^2\)). Therefore, it can easily be simulated as follows:

```
set.seed(1)
= RW(gamma2 = 0.01) + DR(omega = 0.001) + WN(sigma2 = 1)
model = gen_gts(model, n = 10^3)
Yt plot(Yt)
```

It is also possible to retrieve and visualize the three latent used to construct such state-space model using the function `gen_lts()`

instead of `gen_gts()`

, as follows:

```
set.seed(1)
= RW(gamma2 = 0.01) + DR(omega = 0.001) + WN(sigma2 = 1)
model = gen_lts(model, n = 10^3)
Yt plot(Yt)
```

Consider another example, let us suppose that different AR(1) processes are present in a state-space model. The syntax to insert “k” of these models into the state-space model is `k*AR1()`

. So, for example, the sum of three AR1 models, a random walk and a white noise process can be given by a simple expression: `3*AR1()+RW()+WN()`

.

Examples of simulating such models are generated below.

```
# Generate a ARMA(2,1) + WN()
=
arma_wn_model ARMA(ar = c(0.9, -0.5), ma = 0.3, sigma2 = 1) +
WN(sigma = 4)
= gen_gts(n = n, model = arma_wn_model)
arma_wn_sim
# Plot simulation of ARMA(2,1) + WN()
plot(arma_wn_sim)
```

As mentioned earlier, `simts`

provides a function specifically designed to generate and represent latent time series models: `gen_lts()`

. This provides users the option to visualize a breakdown of the underlying processes by applying the `plot()`

function on the result of `gen_lts()`

.

```
# Generate a SARMA() + WN()
=
sarma_wn_model SARMA(ar = 0, ma = 0, sar = 0.98, sma = 0, s = 10, sigma2 = 1) +
WN(sigma2 = 1)
= gen_lts(n = 10^3, model = sarma_wn_model)
sarma_wn_sim
# Plot simulation of SARMA() + WN()
plot(sarma_wn_sim)
```

To better visualize the contribution to each process by using the sam range on the “y-axis.” This can be done with the option `fixed_range = TRUE`

as follows:

`plot(sarma_wn_sim, fixed_range = TRUE)`

In this section, we will briefly show some of the `simts`

package functionalities that can be applied to basic time series analysis. These functionalities are illustrated through example on the following four datasets (stored in `simts`

):

- hydro: This time series contains the monthly precipitation from 1907 and going to 1972 for total of 781 observations taken from Hipel and McLeod (1994).
- savingrt: This dataset contains the US personal saving (after removing seasonal trends) which represent the percentage of income saved from the disposable personal income. This time series with frequency 12 starting in year 1959 and going to 2016 for a total of 691 observations.
- Nile: This time series contains the measurements of the annual flow of the river Nile at Aswan (formerly Assuan), 1871–1970, in \(10^8 m^3\) taken from Table 1 of Cobb (1978).

The code below shows how to setup a time series as a `gts()`

object. Here, we take samples from each dataset at a rate of `freq`

ticks per sample. By applying `plot()`

on the result of a `gts()`

object, we can observe a simple visualization of our data.

Frist, we consider the `hydro`

dataset. The code below shows how to construct a `gts`

object and plot the resulting time series.

```
# Load hydro dataset
data("hydro")
# Simulate based on data
= gts(as.vector(hydro), start = 1907, freq = 12, unit_ts = "in.",
hydro unit_time = "year", name_ts = "Precipitation",
data_name = "Precipitation data from Hipel et al., (1994)")
# Plot hydro simulation
plot(hydro)
```

Using the object we created we can now compute its autocorrelation function using the `auto_corr()`

function as follows:

```
# Compare the standard and robust ACF
compare_acf(hydro)
```

This plot shows that no apparent autocorrelation exists when using the standard estimator of the ACF (left) but the picture changes compeltely when using the robust estimator (right). There therefore appears to be some possible contamination in the data and, if we wanted to estimate a model for the data, we would probably opt for a robust estimator. For this we can use the RGMWM to estimate an AR(1) model which could be a possible candidate to explain the robust ACF pattern.

```
= estimate(AR(2), hydro, method = "rgmwm")
model_hydro $mod$estimate model_hydro
```

```
## Estimates
## AR 0.42983749
## AR -0.04681091
## SIGMA2 0.10528161
```

The estimated value of the autoregressive parameter appears to confirm that there exists some autocorrelation in the data.

Similarly to the first dataset, we now consider the `savingrt`

time series:

```
# Load savingrt dataset
data("savingrt")
# Simulate based on data
= gts(as.vector(savingrt), start = 1959, freq = 12, unit_ts = "%",
savingrt name_ts = "Saving Rates", data_name = "US Personal Saving Rates",
unit_time = "year")
# Plot savingrt simulation
plot(savingrt)
```