The **dsmmR** R package allows the user to estimate,
simulate and define different Drifting semi-Markov model (DSMM)
specifications.

```
# Install the released version from CRAN
install.packages('dsmmR')
# Or the development version from GitHub
# install.packages("devtools")
::install_github("Mavrogiannis-Ioannis/dsmmR") devtools
```

The main functions of **dsmmR** are the following:

`fit_dsmm()`

: estimate a DSMM (parametric or non-parametric estimation is possible).`parametric_dsmm()`

: define a parametric DSMM.`nonparametric_dsmm()`

: define a non-parametric DSMM.`simulate()`

: simulate a sequence from a DSMM.`get_kernel()`

: obtain the Drifting semi-Markov kernel.

Drifting semi-Markov models are best suited to capture non-homogeneities which evolve in a linear (or polynomial) way. For example, through this approach we account for non-homogeneities that occur from the intrinsic evolution of the system or from the interactions between the system and the environment.

For a detailed introduction in Drifting semi-Markov models consider
the documentation through `?dsmmR`

.

For an extensive description of this approach, consider visiting the complete documentation of the package on the official CRAN page.

The easiest way to use **dsmmR** is through the main
function `dsmm_fit()`

in the non-parametric case. This
function can estimate a Drifting semi-Markov model from a sequence of
states (i.e. a character vector in R). Example data is included in the
package, defined in the DNA sequence `lambda`

. Also some
parameters need to be specified before using `dsmm_fit()`

,
most notably the polynomial *degree* and the model of our choice.
The model is chosen by defining whether the sojourn times *f* and
the transition matrices *p* are drifting or not.

```
# Loading the package
library(dsmmR)
# Obtaining the sequence
data("lambda", package = "dsmmR")
<- c(lambda)
sequence
# Obtaining the states
<- sort(unique(sequence))
states
# Defining the polynomial degree
<- 1 # we define a linear evolution in time (state jumps of the embedded Markov chain)
degree
# Defining the model
<- TRUE # sojourn time distributions are drifting in time (state jumps of the EMC)
f_is_drifting <- FALSE # transition matrices are not drifting in time (state jumps of the EMC)
p_is_drifting # When both f and p are drifting, we have Model 1.
# Fitting the Drifting semi-Markov model
<- fit_dsmm(sequence = sequence,
fitted_model states = states,
degree = degree,
f_is_drifting = f_is_drifting,
p_is_drifting = p_is_drifting)
```

For more details about the estimation, consider viewing the extended
documentation through `?fit_dsmm`

.

When defining a DSMM object we need to input parameters like the
polynomial degree, the state space, the DSMM size (length of the
embedded Markov chain), the sojourn times *f*, the transition
matrices *p* and more.

For more information, consider the documentation through
`?parametric_dsmm`

and `?nonparametric_dsmm`

.

After fitting a DSMM (or defining it through
`nonparametric_dsmm()`

or `parametric_dsmm()`

), we
can simulate a sequence from that DSMM. This is pretty
straightforward:

`<- simulate(fitted_model) sim_seq `

Since we follow an object oriented approach, providing the previous
object `fitted_model`

is the only necessary attribute.

For more information, consider the documentation through
`?simulate.dsmm`

.

In order to account for the large dimension of the DSM kernel, a separate function was necessary. You can obtain the DSM kernel through the command:

`<- get_kernel(fitted_model) kernel `

The dimensionality of the DSM kernel can be reduced further through the attributes of the function.

For more information, consider the documentation through
`?get_kernel`

.

Regarding semi-Markov models, the book Semi-Markov Chains and Hidden Semi-Markov Models toward Applications gives a good overview of the topic and also combines the flexibility of the semi-Markov chain with the known advantages of hidden semi-markov models.

If you are not familiar with Drifting Markov models, they were first introduced in Drifting Markov models with Polynomial Drift and Applications to DNA Sequences, while a comprehensive overview is provided in Reliability and Survival Analysis for Drifting Markov Models: Modeling and Estimation.

For third parties wishing to contribute to the software, or to report issues or problems about the software, they can do so directly through the development github page of the package.

Automated tests are in place in order to aid the user with any false
input made and, furthermore, to ensure that the functions used return
the expected output. Moreover, through strict automated tests, it is
made possible for the user to properly define their own
`dsmm`

objects and make use of them with the generic
functions of the package.

If you are in need of support, please contact the maintainer at mavrogiannis.ioa@gmail.com.

Barbu, V. S., Limnios, N. (2008). Semi-Markov Chains and Hidden Semi-Markov Models Toward Applications - Their Use in Reliability and DNA Analysis. New York: Lecture Notes in Statistics, vol. 191, Springer.

Vergne, N. (2008). Drifting Markov models with Polynomial Drift and Applications to DNA Sequences. Statistical Applications in Genetics Molecular Biology 7 (1).

Barbu V. S., Vergne, N. (2019). Reliability and survival analysis for drifting Markov models: modelling and estimation. Methodology and Computing in Applied Probability, 21(4), 1407-1429.