  Let $A$ be a (left and right) Noetherian ring that is semiperfect. Let $e$ be
an idempotent of $A$ and consider the ring $\Gamma:=(1-e)A(1-e)$ and the
semi-simple right $A$-module $S_e : = eA/e{\rm rad}A$. In this paper, we
investigate the relationship between the global dimensions of $A$ and $\Gamma$,
by using the homological properties of $S_e$. More precisely, we consider the
Yoneda ring $Y(e):={\rm Ext}^*_A(S_e,S_e)$ of $e$. We prove that if $Y(e)$ is
artinian of finite global dimension, then $A$ has finite global dimension if
and only if so is $\Gamma$. We also investigate the situation where both
$A,\Gamma$ have finite global dimension. When $A$ is Koszul and finite
dimensional, this implies that $Y(e)$ has finite global dimension. We end the
paper with a reduction technique to compute the Cartan determiant of artin
algebras. We prove that if $Y(e)$ has finite global dimension, then the Cartan
determinants of $A$ and $\Gamma$ coincide. This provides a new way to approach
the long-standing Cartan determinant conjecture.
