Hi all, I am trying to fit full quadratic model i.e liear + square + interaction terms to my data (216 data points), so in my case coefficients are 28 (6 parameters). When i look into summary lm() of model all cross terms coefficients are NA. What does this mean? Is the regression is not able to solve for these coefficients due to some reason. Or does this mean that i should not consider these terms. Regards,
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