  A combinatorial design is a family of sets that are almost disjoint, which is
applied in pseudo random number generations and randomness extractions. The
parameter, $\rho$, quantifying the overlap between the sets within the family,
is directly related to the length of a random seed needed and the efficiency of
an extractor. Nisan and Wigderson proposed an explicit construction of designs
in 1994. Later in 2003, Hartman and Raz proved a bound of $\rho\le e^2$ for the
Nisan-Wigderson construction in a limited parameter regime. In this work, we
prove a tighter bound of $\rho<e$ with the entire parameter range by slightly
refining the Nisan-Wigderson construction. Following the block idea used by
Raz, Reingold, and Vadhan, we present an explicit weak design with $\rho=1$.
