# How to use optim in R

A friend of mine asked me the other day how she could use the function optim in R to fit data. Of course, there are built-in functions for fitting data in R and I wrote about this earlier. However, she wanted to understand how to do this from scratch using `optim`

.

The function `optim`

provides algorithms for general-purpose optimisations and the documentation is perfectly reasonable, but I remember that it took me a little while to get my head around how to pass data and parameters to optim. Thus, here are two simple examples.

I start with a linear regression by minimising the residual sum of squares and discuss how to carry out a maximum likelihood estimation in the second example.

# Minimise residual sum of squares

I start with an x-y data set, which I believe has a linear relationship and therefore I’d like to fit y against x by minimising the residual sum of squares.

```
dat=data.frame(x=c(1,2,3,4,5,6),
y=c(1,3,5,6,8,12))
```

Next, I create a function that calculates the residual sum of square of my data against a linear model with two parameter. Think of `y = par[1] + par[2] * x`

.

```
min.RSS <- function(data, par) {
with(data, sum((par[1] + par[2] * x - y)^2))
}
```

Optim minimises a function by varying its parameters. The first argument of `optim`

are the parameters I’d like to vary, `par`

in this case; the second argument is the function to be minimised, `min.RSS`

. The tricky bit is to understand how to apply `optim`

to your data. The solution is the `...`

argument in `optim`

, which allows me to pass other arguments through to `min.RSS`

, here my data. Therefore I can use the following statement:

`(result <- optim(par = c(0, 1), fn = min.RSS, data = dat))`

```
## $par
## [1] -1.266846 2.028620
##
## $value
## [1] 2.819048
##
## $counts
## function gradient
## 89 NA
##
## $convergence
## [1] 0
##
## $message
## NULL
```

Let me plot the result:

```
plot(y ~ x, data = dat, main="Least square regression")
abline(a = result$par[1], b = result$par[2], col = "red")
```

Great, this looks reasonable. How does it compare against the built in linear regression in R?

`lm(y ~ x, data = dat)`

```
##
## Call:
## lm(formula = y ~ x, data = dat)
##
## Coefficients:
## (Intercept) x
## -1.267 2.029
```

Spot on! I get the same answer.

# Maximum likelihood

In my second example I look at count data and I would like to fit a Poisson distribution to this data.

Here is my data:

```
obs = c(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 17, 42, 43)
freq = c(1392, 1711, 914, 468, 306, 192, 96, 56, 35, 17, 15, 6, 2, 2, 1, 1)
x <- rep(obs, freq)
plot(table(x), main="Count data")
```

To fit a Poisson distribution to `x`

I don’t minimise the residual sum of squares, instead I maximise the likelihood by varying its parameter \(\lambda\).

The likelihood function is given by:

`lklh.poisson <- function(x, lambda) lambda^x/factorial(x) * exp(-lambda)`

and the sum of the log-liklihood function is:

```
log.lklh.poisson <- function(x, lambda){
-sum(x * log(lambda) - log(factorial(x)) - lambda)
}
```

By default `optim`

searches for parameters, which minimise the function `fn`

. In order to find a maximium, all I have to do is change the sign of the function, hence `-sum(...)`

.

`optim(par = 2, log.lklh.poisson, x = x)`

```
## Warning in optim(par = 2, log.lklh.poisson, x = x): one-dimensional optimization by Nelder-Mead is unreliable:
## use "Brent" or optimize() directly
```

```
## $par
## [1] 2.703516
##
## $value
## [1] 9966.067
##
## $counts
## function gradient
## 24 NA
##
## $convergence
## [1] 0
##
## $message
## NULL
```

Ok, the warning message tells me that I shoud use another optimisation algorithm, as I have a one dimensional problem - a single parameter. Thus, I follow the advise and get:

`optim(par = 2, fn = log.lklh.poisson, x = x, method = "Brent", lower = 2, upper = 3)`

```
## $par
## [1] 2.703682
##
## $value
## [1] 9966.067
##
## $counts
## function gradient
## NA NA
##
## $convergence
## [1] 0
##
## $message
## NULL
```

It’s actually the same result. Let’s compare the result to `fitdistr`

, which uses maximum liklihood as well.

```
library(MASS)
fitdistr(x, "Poisson")
```

```
## lambda
## 2.70368239
## (0.02277154)
```

No surprise here, it gives back the mean, which is the maximum likelihood parameter.

`mean(x)`

`## [1] 2.703682`

For more information on optimisation and mathematical programming with R see the CRAN Task View on this subject.

# Session Info

`sessionInfo()`

```
## R version 3.6.2 (2019-12-12)
## Platform: x86_64-apple-darwin15.6.0 (64-bit)
## Running under: macOS Catalina 10.15.3
##
## Matrix products: default
## BLAS: /Library/Frameworks/R.framework/Versions/3.6/Resources/lib/libRblas.0.dylib
## LAPACK: /Library/Frameworks/R.framework/Versions/3.6/Resources/lib/libRlapack.dylib
##
## locale:
## [1] en_GB.UTF-8/en_GB.UTF-8/en_GB.UTF-8/C/en_GB.UTF-8/en_GB.UTF-8
##
## attached base packages:
## [1] stats graphics grDevices utils datasets methods base
##
## other attached packages:
## [1] MASS_7.3-51.5
##
## loaded via a namespace (and not attached):
## [1] compiler_3.6.2 magrittr_1.5 bookdown_0.17 tools_3.6.2
## [5] htmltools_0.4.0 yaml_2.2.0 Rcpp_1.0.3 stringi_1.4.5
## [9] rmarkdown_2.1 blogdown_0.17 knitr_1.27 stringr_1.4.0
## [13] digest_0.6.23 xfun_0.12 rlang_0.4.4 evaluate_0.14
```