Correlation and Regressssion - Agricultural Statistical Data Analysis Using Stata - page 198

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nrate

Figure 10.12 Plotted data of onion yield based on nitrogen fertilizer rate.

Open the dataset Onion Fertility 2005.dta and look at the data for
yield graphically with the following command:

Twoway ( scatter yield nrate)

Look at the graph and the data appear to have a somewhat curved
shape. This gives us a clue as to how the data should be handled
(Figure 10.12). Enter the following regression command and look at
the output:

regress yield nrate c.nrate#c.nrate

Source | SS df MS Number of obs = 36
---------+-------------------------- F( 2, 33) = 99.40
Model | 48601.884 2 24300.942 Prob > F = 0.0000
Residual |8067.45567 33 244.468354 R-squared = 0.8576
---------+--------------------------- Adj R-squared = 0.8490
Total |56669.3397 35 1619.12399 Root MSE = 15.635

----------------------------------------------------------------
yield | Coef. Std. Err. t P>|t| [95% Conf. Interval]
---------+------------------------------------------------------
nrate |.9462218 .0945669 10.01 0.000 .7538241 1.13862
|
c.nrate#
c.nrate |-.0019585 .0003058 -6.40 0.000 -.0025806 -.0013364
|
_cons | 38.53506 5.759268 6.69 0.000 26.81774 50.25238
----------------------------------------------------------------