let inline averageSquareError estimated actual = 
    Array.zip estimated actual
    |> Array.averageBy (fun (w,o) -> pown (o - w) 2)

//let estimated = [|1.0; 2.0; 3.0; 4.0; 5.0|]
//let actual    = [|1.1; 2.2; 2.8; 4.0; 5.1|]

//let err = averageSquareError estimated actual
//val err : float = 0.02