面板help中的例子，不都是多因素的吗？

Examples

    Setup
        . webuse nlswork
        . xtset idcode

    Between-effects model
        . xtreg ln_w grade age c.age#c.age ttl_exp c.ttl_exp#c.ttl_exp tenure c.tenure#c.tenure 2.race not_smsa
            south, be

    Additional setup if not using factor variables
        . generate age2 = age^2
        . generate ttl_exp2 = ttl_exp^2
        . generate tenure2 = tenure^2
        . generate byte black = (race==2)

    Between-effects model same as above, but not using factor variables
        . xtreg ln_w grade age* ttl_exp* tenure* black not_smsa south, be

    Fixed-effects model
        . xtreg ln_w grade age c.age#c.age ttl_exp c.ttl_exp#c.ttl_exp tenure c.tenure#c.tenure 2.race not_smsa
            south, fe

    Fixed-effects model with robust variance
        . xtreg ln_w grade age c.age#c.age ttl_exp c.ttl_exp#c.ttl_exp tenure c.tenure#c.tenure 2.race not_smsa
            south, fe vce(robust)

    Random-effects model
        . xtreg ln_w grade age c.age#c.age ttl_exp c.ttl_exp#c.ttl_exp tenure c.tenure#c.tenure 2.race not_smsa
            south, re theta

    Random-effects model using maximum likelihood estimator
        . xtreg ln_w grade age c.age#c.age ttl_exp c.ttl_exp#c.ttl_exp tenure c.tenure#c.tenure 2.race not_smsa
            south, mle

    Population-averaged model
        . xtreg ln_w grade age c.age#c.age ttl_exp c.ttl_exp#c.ttl_exp tenure c.tenure#c.tenure 2.race not_smsa
            south, pa