I'm trying to make a parameter be sampled according to a Gaussian Random Walk. In R the code would look like this:
#simulate a Gaussian random walk
#N : number of steps
#x0 : initial offset
#mu : drift velocity
#variance : step size
Gauss_RandomWalk <- function(N, x0, mu, variance) {
z <- cumsum(rnorm(n=N, mean=mu, sd=sqrt(variance)))
t <- 1:N
x <- (x0 + t*mu + z)
return(x)
}
Actually, results when setting x0=0.
, mu=0.
, variance=0.035**2
look good and reasonable:
[1] 0.040703269 0.009159686 0.052360030 0.059352074 0.092739218 0.098240752 0.113957813 0.064187776
[9] 0.062757728 0.063948224 0.034591074 0.004828493 0.019809969 0.032135111 0.025692763 -0.031678858
[17] -0.048033007 -0.020708105 -0.032231674 0.004917305 0.030961430 0.099054042 0.043441737 -0.010513085
Whenever I'm trying to do it in Stan, like for example according to what is represented here :
// model to be fitted
model {
sqrtQ ~ student_t(2, 0, 0.035);
// here we define the random walk for the log_Rt parameter
log_Rt[1] ~ normal(0. , 0.035);
for (t in 2:number_days) {
log_Rt[t] ~ normal(log_Rt[t-1], sqrtQ);
}
print(log_Rt);
(...)
}
Results aren't that good. Look for example the jump -0.839043 -> 1.91956
which is about 85 times the standard deviation and statistically would be impossible... But why do such jumps occur?
Chain 1: [0.382303,-0.489057,0.33374,-0.839043,1.91956,0.249953,-1.88793,1.61106,1.11189,-0.725063,-0.513174,1.79012,1.57758,0.75819,0.525524,1.44762,1.19118,0.485563,-1.48318,-1.36389,1.96355,0.321416,-0.365132,-0.644287,0.0981577,1.02943,-1.27993,-1.98085,-1.75191,-1.76489,1.60888,1.48925,-0.0452427,-0.92583,-1.21594,-0.906329,-0.700237,-0.208039,0.493656,0.490295,-1.61091,1.94587,0.758567,-1.02318,-1.92659,-0.999492,1.78042,0.214125,-0.0158054,-0.753422, .......
EDIT: I tried as well:
// model to be fitted
model {
// here we define the random walk for the log_Rt parameter
log_Rt[1] ~ normal(0. , 0.035);
for (t in 2:number_days) {
log_Rt[t] ~ normal(log_Rt[t-1], 0.035);
}
print(log_Rt);
(...)
}
But things do not change at all.
The normal function you use actually tries to estimate the probability of log_Rt
and therefor produces strange results. You could use generate quantities to do something similar to what you try in r:
require(rstan)
#> Loading required package: rstan
#> Loading required package: StanHeaders
#> Loading required package: ggplot2
#> rstan (Version 2.21.2, GitRev: 2e1f913d3ca3)
#> For execution on a local, multicore CPU with excess RAM we recommend calling
#> options(mc.cores = parallel::detectCores()).
#> To avoid recompilation of unchanged Stan programs, we recommend calling
#> rstan_options(auto_write = TRUE)
scode<-"
generated quantities {
real y[10];
y[1] = normal_rng(0, .035);
for (t in 2:10) {
y[t] = normal_rng(y[t-1], 0.035);
}}"
m <- stan_model(model_code = scode)
f<-sampling(m, iter=1, chain=1, algorithm='Fixed_param')
#>
#> SAMPLING FOR MODEL '82a947a4909cd091541c3476a3eab1f0' NOW (CHAIN 1).
#> Chain 1: Iteration: 1 / 1 [100%] (Sampling)
#> Chain 1:
#> Chain 1: Elapsed Time: 0 seconds (Warm-up)
#> Chain 1: 1.4e-05 seconds (Sampling)
#> Chain 1: 1.4e-05 seconds (Total)
#> Chain 1:
extract(f,'y')
#> $y
#>
#> iterations [,1] [,2] [,3] [,4] [,5] [,6]
#> [1,] 0.06017424 0.07683703 0.08551969 0.04428875 -0.001519853 0.000953893
#>
#> iterations [,7] [,8] [,9] [,10]
#> [1,] 0.008921459 -0.06060675 -0.04087786 -0.02217786
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