  Kuroda's formula relates the class number of a multi-quadratic number field
$K$ to the class numbers of its quadratic subfields $k_i$. A key component in
this formula is the unit group index $Q(K) = [\mathcal{O}_{K}^{\times}:
\prod_i\mathcal{O}_{k_i}^{\times}]$. We study how $Q(K)$ behaves on average in
certain natural families of totally real biquadratic fields $K$ parametrized by
prime numbers.
