*Thyne, Clayton
*Intro to Stats, Chapter 11 examples

	clear
	set mem 80m
	cd [your directory]

*  NOTE: the above command asks you to change the directory.  The directory will depend on how you've set up
*  the folders on your computer.  For example, my command is...
*  cd F:\teaching\Fall_2008\PS572-401-baby_stats\do_file_directories\chapter11
*  This is necessary so you know where stata is saving things.

****Part 1: examining the line of best fit
	use "http://www.uky.edu/~clthyn2/intro_stats/GPA-ACT.dta"

*First, let's add some data for ACT
	gen ACT=.
	replace ACT = 28 in 1
	replace ACT = 27 in 2
	replace ACT = 26 in 3
	replace ACT = 29 in 4
	replace ACT = 24 in 5
	replace ACT = 31 in 6
	replace ACT = 25 in 7
	replace ACT = 35 in 8
	replace ACT = 25 in 9
	replace ACT = 24 in 10
	replace ACT = 26 in 11
	replace ACT = 21 in 12
	drop if ACT==.

*Now, let's take a look at the data.  NOTE: The Y axis should always
*be the DV, which we always write first in stata.
	scatter GPA ACT

*Now, add a line of 'best fit'
	graph twoway lfit GPA ACT || scatter GPA ACT
	graph twoway lfit GPA ACT || scatter GPA ACT, mlabel(school)

****Part 2: the OLS model example
*Note: Minor differences are due to rounding errors
	regress GPA ACT
	*Note the significant P value for ACT (.000).  
	*Stata is using the t table with 11 degrees of freedom.
	*Note the ANOVA table and significant F test for the model
	*Note R-squared and adjusted R-squared

****Part 3: examining regression further
	regress GPA ACT
	predict yhat
	scatter yhat ACT
	predict ehat, resid
	scatter ehat ACT
	gen observed=yhat+ehat

****Part 4: the lowess curve
	graph twoway lfit GPA ACT || scatter GPA ACT, saving(lfit, replace)
	graph twoway lowess GPA ACT || scatter GPA ACT, saving(lowess, replace)
	graph combine lfit.gph lowess.gph

*As a better example, let's make one of observations an outlier
	replace ACT = 15 in 11
	graph twoway lfit GPA ACT || scatter GPA ACT, saving(lfit, replace)
	graph twoway lowess GPA ACT || scatter GPA ACT, saving(lowess, replace)
	graph combine lfit.gph lowess.gph