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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