The variables hhsize (household size} and hhsizeZ [square ot household size) are added to Model 1 from above. Here is the Stata output: . *Model 2 . gen hhsize2=hhsize\"2 . sum hhsize hhsizez Variable Obs Mean Std. Dev. Min Max hhsize 499 4.480962 2.153063 1 15 hhsize2 499 24.70541 24.40701 1 225 . reg earning femalehead hhsize hhsize2 Source SS df MS Number of obs = 499 F(3, 495) = 26.14 Model 50982302 3 16994100.7 Prob > F = 0.0000 Residual 321773113 495 650046.694 Rsquared = 0.1368 Adj Rsquared = 0.1315 Total 372755415 498 748504.85 Root MSE = 806-25 earning Coef. Std. Err. t P>|t| [95% Conf. Interval] femalehead -303.2039 92.90906 -3.26 0.001 485.7487 -120.6592 hhsize 270.8402 56.80166 4.77 0.000 382.4423 159.2381 hhsize2 12.35874 5.002342 2.47 0.014 2.530302 22.18718 _cons 2595.94 149.3195 17.39 0.000 2302.562 2889.318