# Bootstrap implementation

## Computation of standard errors

Objects of maxlogL class (outputs frommaxlogL and maxlogLreg) stores the estimated parameters of probability density/mass functions by Maximum Likelihood. The variance-covariance matrix is computed from Fisher information matrix, which is obtained by means of the Inverse Hessian matrix of estimators:

$\begin{equation} Var(\hat{\boldsymbol{\theta}}) = I^{-1}(\hat{\boldsymbol{\theta}}) = C(\hat{\boldsymbol{\theta}}), \end{equation}$

where $$I(\hat{\boldsymbol{\theta}})$$ is the Fisher Information Matrix. Hence, the standard errors can be calculated as the square root of the diagonal elements of matrix $$C$$, as follows:

$\begin{equation} SE(\hat{\boldsymbol{\theta}}) = \sqrt{C_{jj}(\hat{\boldsymbol{\theta}})}, \end{equation}$

To install the package, type the following commands:

if (!require('devtools')) install.packages('devtools')
devtools::install_github('Jaimemosg/EstimationTools', force = TRUE)

In EstimationTools Hessian matrix is computed in the following way:

• Firstly, it is estimated through optim, with option hessian = TRUE in maxlogL function.
• If the previous implementation fails, it is calculated with hessian function from numDeriv package.
• If both of the previous methods fail, then standard errors are computed by bootstrapping with the function bootstrap_maxlogL.

Additionally, EstimationTools allows implementation of bootstrap for standard error estimation, even if the Hessian computation does not fail.

## Standard Error with maxlogL function

Lets fit the following distribution:

\begin{aligned} X &\sim N(\mu, \:\sigma^2) \\ \mu &= 160 \quad (\verb|mean|) \\ \sigma &= 6 \quad (\verb|sd|) \end{aligned}

The following chunk illustrates the fitting with Hessian computation via optim:

library(EstimationTools)

x <- rnorm(n = 10000, mean = 160, sd = 6)
theta_1 <- maxlogL(x = x, dist = 'dnorm', control = list(trace = 1),
fixed = list(mean = 160))
#>   0:     43081.027:  1.00000
#>   1:     32422.767:  2.00000
#>   2:     32170.782:  1.91451
#>   3:     32020.763:  1.74935
#>   4:     32010.038:  1.78651
#>   5:     32009.839:  1.78219
#>   6:     32009.839:  1.78204
#>   7:     32009.839:  1.78205
summary(theta_1)
#> _______________________________________________________________
#> Optimization routine: nlminb
#> Standard Error calculation: Hessian from optim
#> _______________________________________________________________
#>        AIC      BIC
#>   64019.68 64019.68
#> _______________________________________________________________
#>    Estimate  Std. Error Z value Pr(>|z|)
#> sd   5.94200    0.04202   141.4   <2e-16 ***
#> ---
#> Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
#> _______________________________________________________________
#> Note: p-values valid under asymptotic normality of estimators
#> ---

## Hessian
print(theta_1$fit$hessian)
#>          [,1]
#> [1,] 566.4547

## Standard errors
print(theta_1$fit$StdE)
#>  0.04201626
print(theta_1$outputs$StdE_Method)
#>  "Hessian from optim"

Note that Hessian was computed with no issues. Now, lets check the aforementioned feature in maxlogL: the user can implement bootstrap algorithm available in bootstrap_maxlogL function. To illustrate this, we are going to create another object theta_2:

# Bootstrap
theta_2 <- maxlogL(x = x, dist = 'dnorm', control = list(trace = 1),
fixed = list(mean = 160))
#>   0:     43081.027:  1.00000
#>   1:     32422.767:  2.00000
#>   2:     32170.782:  1.91451
#>   3:     32020.763:  1.74935
#>   4:     32010.038:  1.78651
#>   5:     32009.839:  1.78219
#>   6:     32009.839:  1.78204
#>   7:     32009.839:  1.78205
bootstrap_maxlogL(theta_2, R = 200)
#>
#> ...Bootstrap computation of Standard Error. Please, wait a few minutes...
#>
#>
#>  --> Done <---
summary(theta_2)
#> _______________________________________________________________
#> Optimization routine: nlminb
#> Standard Error calculation: Bootstrap
#> _______________________________________________________________
#>        AIC      BIC
#>   64019.68 64019.68
#> _______________________________________________________________
#>    Estimate  Std. Error Z value Pr(>|z|)
#> sd   5.94200    0.04478   132.7   <2e-16 ***
#> ---
#> Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
#> _______________________________________________________________
#> Note: p-values valid under asymptotic normality of estimators
#> ---

## Hessian
print(theta_2$fit$hessian)
#>          [,1]
#> [1,] 566.4547

## Standard errors
print(theta_2$fit$StdE)
#>  0.04478343
print(theta_2$outputs$StdE_Method)
#>  "Bootstrap"

Notice that Standard Errors calculated with optim ($$0.042016$$) and those calculated with bootstrap implementation ($$0.044783$$) are approximately equals, but no identical.