Indeed, and this constraint is not trivial. The "solution" option in proc glm uses dummy coding which renders the parameter estimates for main effects uninterpretable as main effects (philosophical question of whether one should interpret main effects notwithstanding). If regression estimates are desired for the main effects in a 2x2, orthogonal contrast coding is required. Normally when running an experiment one is not interested in regression coefficient. However there are cases where the coeffiicents are desireable -- for example when testing mediation and estimating indirect effects. In such cases, DONT use the 'solution' option in proc glm.
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