A number of procedures for testing adequacy of polynomial approximations to growth curves based on Rao's test for additional information, Grizzle and Allen's test --or univariate t-tests were compared using data simulated from quadratic models. Quadratic models were indicated as adequately fitting the data in 95.10k0.10 percent of analyses when the degree of the approximating polynomial was determined by the lowest-order significant coefficient (P=0.05) that was followed by two successive nonsignificant ones according to separate t-tests. Procedures based on the Grizzle and Allen test and modifications of it indicated quadratic models in 86.70k0.41 to 95.34 f 0.21 percent of analyses depending on error structure, variance and the number of coefficients analysed together. The t-test would be preferred in practice as its performance did not depend on the error structure or variance.
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