stiffness factor will be EI/L

they are term's of stiffness in the elasticity 

Depend on boundary condition too. 

To get the deflection of a member you need:

Modulus of Elasticity (E)

Ix, Iy and J for combined bending and torsion

boundary conditions

loads and distribution.

Standard textbooks on mechanics of materials give full solutions for loading in the elastic range.

The Stiffness factor k= EI/L

where:

E- Modulus of elasticity of the material which specify the properties of the materials

I - Moment of inertia depend upon the shape and size of the cross section 

L - Clear Span of the Beam