st: Age-specific reference intervals (xrigls) - no equation for SD?

[Sofia Ramiro <sofiaramiro@hotmail.com>]

Dear Statalist members, 

I am trying to define age-specific reference intervals with the command xrigls. 

However, when running it, for some of the variables for which I want to define the age intervals I get an equation for the mean (with the corresponding power), but no equation for the SD - example below. I don't understand why. Can any of you explain this?
Are the values derived without the SD equation also right?
I don't understand why in the 1st table (FP powers) the change appears always as 0. I inspected the data and there is variation in this variable and throughout the different ages. 

Thanks

Sofia



. xrigls schober15 age if sample==1, fp(m:df 4,s:df 2) centile(2.5 10 25 75 90 97.5) detail

          --- FP Powers ---
Cycle     Mean       SD          Deviance     Change  Residual SS
-----------------------------------------------------------------
0         1                      1217.262      0.000   509.4298
1         1                      1217.262      0.000   509.4298
2         1                      1217.262      0.000   509.4298

Final deviance =  1217.262 (393 observations).
Power(s) for mean curve = 1.

Regression for mean curve
-------------------------
(sum of wgt is   3.4287e+02)

      Source |       SS       df       MS              Number of obs =     393
-------------+------------------------------           F(  1,   391) =   24.05
       Model |  31.3308787     1  31.3308787           Prob > F      =  0.0000
    Residual |  509.429806   391  1.30288953           R-squared     =  0.0579
-------------+------------------------------           Adj R-squared =  0.0555
       Total |  540.760685   392  1.37949154           Root MSE      =  1.1414

------------------------------------------------------------------------------
   schober15 |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
         age |  -.0203814   .0041562    -4.90   0.000    -.0285528     -.01221
       _cons |   7.191451   .1913316    37.59   0.000     6.815284    7.567619
------------------------------------------------------------------------------

 		 	   		  
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