Again, without memorizing this formula, we can intuitively understand
how its various components contribute to sample size. The greater
the absolute values of za, zb, and σ, and the smaller
the difference in the means, μ1 - μ2, the larger
the n, or sample size, required (see
Table 7–3). This makes sense, as smaller differences in
means between groups would be harder to detect, and greater variability
within the groups would tend to blur intergroup differences. As
in all sample size calculations, the larger the values of za and zb—that
is, the smaller the acceptable type I and type II errors—the
larger the sample size required.