It is clear from Figures 1 -
3 that tests
of the consistency of SBBN, along with constraints on any new physics, will
be data-driven. While D (and/or 3He and/or 7Li)
largely constrain the baryon density and 4He plays a similar
role for 
N
and/or
for 
e,
there is an interplay among
10,
N,
and e,
which is quite sensitive to the adopted abundances. For example, a
lower primordial D/H increases the BBN-inferred value of
10,
leading to a higher predicted primordial
4He mass fraction. If the primordial 4He mass
fraction derived from the data is "low", then a low upper bound on
N (or a
nonzero lower bound on
e)
will be inferred. It is therefore crucial to avoid
biasing any conclusions by underestimating the present uncertainties
in the primordial abundances derived from the observational data.
The four light nuclides of interest, D, 3He, 4He, and 7Li follow very different evolutionary paths in the post-BBN Universe. In addition, the observations leading to their abundance determinations are also very different. Neutral D is observed in absorption in the UV; singly ionized 3He is observed in emission in Galactic HII regions; both singly and doubly ionized 4He are observed in emission via recombinations in extragalactic HII regions; 7Li is observed in absorption in the atmospheres of very metal-poor halo stars. The different histories and observational strategies provide some insurance that systematic errors affecting the inferred primordial abundances of any one of the light nuclides are unlikely to distort the inferred abundances of the others.