/***********
syntax 12.2
***********/
/***************************
Applications of XI command
***************************/
sysuse auto,clear
/***1.1:simple coding: comparing each group to a reference group***/
xi3:reg mpg g.rep78
xi:reg mpg i.rep78

/***1.2: forward difference coding:comparing each group to the next(adjacent) group***/
xi3:reg mpg a.rep78

/***1.3:backward difference coding:comparing each group to the prior adjacent group***/
xi3:reg mpg b.rep78

/***1.4:Helmert coding: comparing each group to the mean of the subsequent groups***/
xi3:reg mpg h.rep78

/***1.5:Reverse Helmert coding: comparing each group to the mean of the previous groups***/
xi3:reg mpg r.rep78

/***1.6:User defined coding***/
keep if rep78>1
tab rep78,sum(mpg) mean
char rep78[user] (1 0 -1 0 \-.5 1 0 -.5 \ -.5 -.5 .5 .5) 	
/***note:(1 0 -1 0):contrast of 1st and 3rd group; (-.5 1 0 -.5):contrast of 2nd group and the average of ///
	1st and 4th group; (-.5 -.5 .5 .5):contrast of the first two and last two groups***/
xi3:reg mpg u.rep78

/***********************************
How to interpret log transformation?
***********************************/
/***Situation 1: exponential model,dependeant variable transformed***/
sysuse auto,clear
hist mpg
gen l_mpg=log(mpg)
scatter mpg price
scatter l_mpg price
hist l_mpg
reg l_mpg price

/***Situation 2: log model: independent variable transformed***/
hist weight
gen l_weight=log(weight)
hist l_weight
reg mpg foreign l_weight

/***Situation 3: Power model, both dependent and independent variable transformed***/
reg l_mpg foreign l_weight

/*******************************************************
How to reg when your dependent variable is a proportion
*******************************************************/
use http://cdph.fsu.edu/sya6933/proportion,clear
sum meals
twoway scatter meals parented,yline(0 1)
reg meals parented
predict mhat
list in 1/10

sum mhat
twoway scatter mhat parented,yline(0 1)

glm meals parented, link(logit) family(binomial) robust nolog
predict mhat2
sum mhat2
twoway scatter mhat2 parented,yline(0 1)

/***********************************
How to apply weights in Stata?
***********************************/
/***pweights: sampling/probability weights***/

clear
input race gpa
1 3.2
1 2.8
1 3.1
1 3.5
1 3.0
2 3.1
2 2.9
2 3.2
2 2.5
2 3.5
3 3.5
3 3.6
3 4
3 3.8
3 3.6
end
list
gen weight=0
replace weight=13.4 if race==1
replace weight=2.4 if race==2
replace weight=1 if race==3
list
mean gpa
mean gpa [pweight=weight]
gen gpa1=gpa*weight
gen N=sum(weight)
gen sum_gpa=sum(gpa1)
display 264.02/84