change bandwidth of npregress kernel

[Julian Schaper]

Dear all,

I am trying to illustrate Engel curves using this command:

 npregress kernel income budgetshare_beef

The red line looks a little too jumpy for my taste. How (in what direction) would I change the bandwidth in order for it to get more linear?

Thanks a lot in advance!

Best,
Julian

[FernandoRios]

Hi Julian
Keep in mind that npregress estimates the bandwidth so that it is the "optimal" one given your data.
If you want to relax this optimality condition, I think it would be simpler to use lpoly

lpoly income budgetshare, bw(##) [kernel($$$)] degree(1)

Degree (1) makes the above command to be a local linear regression, which is the same as npregress does.
HTH

[Julian Schaper]

Thanks for pointing that out!

Just in case I would like to stick to the np regress command, how would I decrease the bandwidth? I struggle to comprehend the options in the help file..

Best,
Julian

[Enrique Pinzon (StataCorp)]

Hi Julian,

You can make the function more linear by increasing the size of the bandwidth. Below is an example of how I would do this using -npregress kernel-

sysuse auto, clear
npregress kernel mpg price
npgraph, name(one, replace)
matrix A = e(bwidth)*5
npregress kernel mpg price, bwidth(A)
npgraph, name(two, replace)
graph combine one two

I used the returned matrix -e(bwidth)- as a starting point and then multiply it by a factor. I fed it into the -bwidth()- option after that.