= Solution
Use two crossed sets of <blocks in experimental design>: volunteers as rows and afternoons as columns. Volunteer blocks control persistent ability, drawing style and prior experience; afternoon blocks control conditions shared that day, including common task difficulty and general practice over time. Neither partition is nested in the other.
\b[Give every volunteer each program twice, and use each program twice each afternoon.] The <row-column design> is then balanced for both block systems. In the additive model
$$
\mathbb E[Y_{vt}]=\mu+\alpha_v+\beta_t+\tau_{d(v,t)},
$$
the program contrasts are <orthogonal> to both centered block spaces. This controls additive volunteer and afternoon effects; it does not automatically eliminate individual learning differences or <carryover effects>.
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