Which variable is NOT helpful in decreasing deflection according to the provided principles?

Increasing span length is indeed the variable that is not helpful in decreasing deflection. In the context of structural engineering, the deflection of a beam or structure is influenced significantly by its span, material properties, and cross-sectional geometry.

When the span length of a beam is increased, it generally results in greater deflection under a given load. This is due to the fact that a longer span can create larger bending moments for the same load, leading to more pronounced deformation. Consequently, increasing the span length typically exacerbates deflection issues.

In contrast, increasing the moment of inertia of a beam's cross-section improves resistance to bending and reduces deflection. Decreasing the span length minimizes the distance over which the beam must carry the load, also leading to reduced deflection. Additionally, increasing the yield strength of the material can enhance the overall load-bearing capacity and resistance to deformation, although the benefit in this context is more about the material’s ability to handle loads without yielding rather than directly reducing deflection.

Thus, among the given options, increasing span length is the only variable that would not assist in reducing deflection, aligning with fundamental principles of structural behavior.