# Looking to prevent overfitting while using DataDrivenDiffEq.jl to estimate the equation for a time dependent diff eq

In this code, we are looking to see if we can use DataDrivenDiffEq.jl to accurate predict an equation for a time dependent differential equation. To test this, we have been creating an equation, adding noise to the solution, and seeing if we can get it back using the package. We have been able to get a good fit, but the equations it gives us back have added terms in them. Here is the code followed by the output.

``````using DataDrivenDiffEq, ModelingToolkit, OrdinaryDiffEq

rng = StableRNG(1000)

# Work on preventing overfitting

function f(u, p, t)
x, y = u
dx = 2.0 * x * y
dy = 1
return [dx, dy]
end

# Setting initial conditions
u0 = [1.0; 0]
tspan = (0.0, 2.0)
dt = 0.0001

# Solving ODE Problem
prob = ODEProblem(f, u0, tspan)
sol = solve(prob, Tsit5(), saveat=dt)

X = sol[:, :] + 0.2 .* randn(rng, size(sol))
ts = sol.t

@variables u[1:2]
u = collect(u)

h = Num[polynomial_basis(u, 2); u]
basis = Basis(h, u)

sampler = DataProcessing(split=0.8, shuffle=true, batchsize=25, rng=rng)
lambdas = exp10.(-10:0.1:0)
opt = STLSQ(lambdas)
res = solve(prob, basis, opt, options=DataDrivenCommonOptions(data_processing=sampler, digits=2))

system = get_basis(res)
params = get_parameter_map(system)

# Displaying results
println(system)
println(params)
display(plot(plot(prob), plot(res), layout=(1, 2)))
``````

Model #basis#311 with 2 equations
States : u u
Parameters : 6
Independent variable: t
Equations
Differential(t)(u) = p₁ + p₃*(u^2) + p₄u + p₅u + p₂*u*u
Differential(t)(u) = p₆

Pair{SymbolicUtils.BasicSymbolic{Real}, Float64}[p₁ => -1.38, p₂ => 1.38, p₃ => -1.6, p₄ => 1.27, p₅ => 1.17, p₆ => 0.99]

Ideally, we should be getting a system of equations that look exactly like what we put in, but it is adding terms. To try to get a better system of equations out, we have decreased dt and limited the equation we get back to a maximum of degree 2. Is there anything else we should do to try to get a more accurate output?

What happens when you adjust the lambda thresholds?

I tried adjusting the upper and lower bounds and adjusting the increment, but it only made the output worse or didn’t affect it at all.

It should decrease the number of terms when increased though?

What parts specifically did you think I should change? I’m new to STLSQ, and I probably just didn’t do what you meant.

It should just require shifting `lambdas = exp10.(-10:0.1:0)`, where if the higher threshold is used the less points you should get.

I tried adjusting it to `lambdas = exp10.(-5:0.1:5)` and it gave this:

Differential(t)(u) = p₁ + p₃*(u^2) + p₄u + p₅u + p₂uu
Differential(t)(u) = p₆ + p₇
(u^2) + p₈
u

Pair{SymbolicUtils.BasicSymbolic{Real}, Float64}[p₁ => -1.81, p₂ => 1.3, p₃ => -2.21, p₄ => 1.47, p₅ => 1.94, p₆ => 0.88, p₇ => -0.24, p₈ => 0.4]

`lambdas = exp10.(0:0.1:5)` gave this:

Differential(t)(u) = p₁ + p₃*(u^2) + p₄u + p₅u + p₂*u*u
Differential(t)(u) = p₆

Pair{SymbolicUtils.BasicSymbolic{Real}, Float64}[p₁ => -1.44, p₂ => 1.37, p₃ => -1.63, p₄ => 1.28, p₅ => 1.28, p₆ => 1.01]

So it seems like it either gets worse or just not any better

Can you open an issue in the repo? We’ll have to test this out a bit.