An Asymptotic Formula for the Number of Balanced Incomplete Block Design   Incidence Matrices
  We identify a relationship between a random walk on a certain Euclidean lattice and incidence matrices of balanced incomplete block designs. We then compute the return probability of the random walk and use it to obtain the asymptotic number of BIBD incidence matrices (as the number of columns increases). Our strategy is similar in spirit to the one used by de Launey and Levin to count partial Hadamard matrices. 
Aaron M. Montgomery
math.CO math.PR