Hi,

Running sobol_sensitivity in DiffEqSensitivity gives some negative values - even while useing the equation ind documents.

```
function f(du,u,p,t)
du[1] = p[1]*u[1] - p[2]*u[1]*u[2]
du[2] = -3*u[2] + u[1]*u[2]
end
u0 = [1.0;1.0]
tspan = (0.0,10.0)
p = [1.5,1.0]
prob = ODEProblem(f,u0,tspan,p)
t = collect(range(0, stop=10, length=200))
s0 = sobol_sensitivity(prob,Tsit5(),t,[[1,5],[0.5,5]],N,0)
Out[8]: 2-element Array{Array{Float64,2},1}:
[NaN 0.507831 … 1.00731 1.00436; NaN 1.92336 … 0.732384 0.730945]
[NaN 0.47214 … 0.676224 0.681525; NaN -1.68656 … 0.879557 0.877603]
s1 = sobol_sensitivity(prob,Tsit5(),t,[[1,5],[0.5,5]],N,1)
Out[9]: 2-element Array{Array{Float64,2},1}:
[NaN 0.39537 … 0.341697 0.343645; NaN -2.06101 … 0.10922 0.106976]
[NaN 0.652815 … 0.00910675 0.00815206; NaN 5.24832 … 0.296978 0.296639]
s2 = sobol_sensitivity(prob,Tsit5(),t,[[1,5],[0.5,5]],N,2)
Out[10]: 1-element Array{Array{Float64,2},1}:
[NaN -0.0596478 … 0.652303 0.657847; NaN -1.84504 … 0.645139 0.620036]
```

for example -0.0596478 in second element of last list.

Is there anyway to check the CI or stdev of the index (not clear form documentation) to see if 0 is included in the index CI - This would be useful to determine if should assume the value of the index to be zero or if I have to rerun the sensitivity analysis with a higher N.

Thanks,

A