Terminal rise velocity of a spherical shaped solid in a liquid obeys the following functional relationship:
U = f(d, W, µ, ρ)
Where, U is the terminal rise velocity, d is the diameter of the solid, W is the apparent weight of the solid, µ is the viscosity of liquid and ρ is the density of liquid. According to Buckingham Π theorem, the number of independent dimensionless variables needed to describe the phenomenon is _______.