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# Re: st: extract part of matrix e(b)

 From Mirko To statalist@hsphsun2.harvard.edu Subject Re: st: extract part of matrix e(b) Date Fri, 22 Jul 2011 19:42:33 +0100

```Hi Jorge,

What if I need to save matrices because I need to do other tests and
calculations?

sysuse auto, clear
set more off
margins, dydx(rep78)
(output omitted)
. matrix b_num = r(b)
. matrix colna b_num = num:
. matrix J_num = r(Jacobian)
. matrix rowna J_num = num:
. margins, dydx(weight) over(rep78)
(output omitted)
. matrix b_den = r(b)
. matrix colna b_den = den:
. matrix J_den = r(Jacobian)
. matrix rowna J_den = den:
. * combine the results
. matrix b = b_num, b_den
. matrix J = J_num \ J_den
. matrix V = J*e(V)*J'
. ereturn post b V
. * check that the combined results match those from the original
. ereturn display
(output omitted)

. nlcom   (R2: [num]_b[2.r]/[den]_b[2.r])  (R3:
[num]_b[3.r]/[den]_b[3.r])  (R4: [num]_b[4.r]/[den]_b[4.r])   (R5
[num]_b[5.r]/[den]_b[5.r])

However, I don't want to use the entire e(b) matrix but just part of
it...Does it work if I save the coefficients as scalars, then I create
a matrix?

Mirko

On 22 July 2011 17:32, Joerg Luedicke <joerg.luedicke@gmail.com> wrote:
> You could grab it from the matrix by addressing row and column
> numbers, for example:
>
> scalar second=b[1,2]
> scalar third=b[1,3]
>
> J.
>
>
> On Fri, Jul 22, 2011 at 12:03 PM, Mirko <mirko.moro@gmail.com> wrote:
>> Dear Statalisters,
>>
>> I wish to extract only some coefficients from the matrix e(b) and I
>> wonder how this can be achieved.
>>
>> Here is a simple example.
>>
>> . regress mpg foreign i.rep78##c.weight headroom
>> (output omitted)
>>
>> . margins, dydx(rep78)
>> (output omitted)
>>
>> . mat b =e(b)
>>
>> This command extracts and saves the entire coefficients matrix e(b) as b.
>>
>> If you type:
>>
>> . mat li b
>>
>> you get:
>>
>> b[1,5]
>>            1b.          2.          3.          4.          5.
>>         rep78       rep78       rep78       rep78       rep78
>> y1           0   .25956254  -.74363551  -.38630819  -6.5437415
>>
>>
>> What should I do if I wish to grab only the second and third
>> coefficients:  .25956254,  -.74363551 instead the all matrix?
>>
>> How this can be done?
>>
>> Mirko
>> *
>> *   For searches and help try:
>> *   http://www.stata.com/help.cgi?search
>> *   http://www.stata.com/support/statalist/faq
>> *   http://www.ats.ucla.edu/stat/stata/
>>
>
> *
> *   For searches and help try:
> *   http://www.stata.com/help.cgi?search
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/
>

*
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```