How do I impute missing values for a single variable using PROC MI?

[ThomasNord]

Hi all,

I am working with forest inventory data where some field plots were not visited in the field. I would like to impute the missing values of forest cover within the plots. I have tried using PROC MI (SAS ver. 9.4), but keep getting the message: "ERROR: Fewer than two analysis variables". In the inserted code NFI2KMCL and ssu form an identifier of the plot, "forest" is wether the plot has been identified as forest (can be 1 or 2) and "A_forest" is the measured forest area, that is sometimes missing and needs to be imputed (values can only be 0 to 0.0706 hectar as the circular plots have a radius of 15 m).

Hope that someone can help me out!

Thomas

data NFI;
input nfi2kmcl ssu $  forest  A_forest;
cards;
2km_6396_588_EUREF89   C   0   0
2km_6396_588_EUREF89   E   0   0
2km_6070_660_EUREF89   E   1   0.070685835
2km_6070_660_EUREF89   G   1   0.018552237
2km_6070_662_EUREF89   A   1   .
2km_6070_662_EUREF89   G   1   .
2km_6070_666_EUREF89   A   1   .
2km_6070_666_EUREF89   G   1   .
2km_6070_672_EUREF89   C   1   0.070685835
2km_6070_672_EUREF89   E   1   0.070685835
2km_6070_672_EUREF89   G   1   0.070685835
2km_6070_688_EUREF89   A   1   0.070685835
2km_6070_688_EUREF89   E   1   0.070685835
2km_6070_688_EUREF89   G   1   0.070685835
2km_6080_524_EUREF89   C   1   0
2km_6080_524_EUREF89   E   1   0.070685835
2km_6080_526_EUREF89   A   1   .
2km_6080_526_EUREF89   G   1   .
2km_6080_528_EUREF89   A   1   .
2km_6080_528_EUREF89   C   1   .
;


proc mi data=NFI seed=501213 nimpute=6 min=0 max=0.070686 out=NFI_out;
	mcmc;
	var A_forest;
	by forest;
run;

[Kurt_Bremser]

What if you let it work over two variables:

data NFI;
input nfi2kmcl :$20. ssu $  forest  A_forest;
cards;
2km_6396_588_EUREF89   C   0   0
2km_6396_588_EUREF89   E   0   0
2km_6070_660_EUREF89   E   1   0.070685835
2km_6070_660_EUREF89   G   1   0.018552237
2km_6070_662_EUREF89   A   1   .
2km_6070_662_EUREF89   G   1   .
2km_6070_666_EUREF89   A   1   .
2km_6070_666_EUREF89   G   1   .
2km_6070_672_EUREF89   C   1   0.070685835
2km_6070_672_EUREF89   E   1   0.070685835
2km_6070_672_EUREF89   G   1   0.070685835
2km_6070_688_EUREF89   A   1   0.070685835
2km_6070_688_EUREF89   E   1   0.070685835
2km_6070_688_EUREF89   G   1   0.070685835
2km_6080_524_EUREF89   C   1   0
2km_6080_524_EUREF89   E   1   0.070685835
2km_6080_526_EUREF89   A   1   .
2km_6080_526_EUREF89   G   1   .
2km_6080_528_EUREF89   A   1   .
2km_6080_528_EUREF89   C   1   .
;

proc mi data=NFI seed=501213 nimpute=6 min=0 max=0.070686 out=NFI_out;
mcmc;
var forest A_forest;
run;

[ThomasNord]

Well, then it works of course, but the intention was to impute only the variable of interest. If I use some other variable just to make it run, that variable will affect the result of the imputation ... at least as far as I understand it.

Thomas

[Kurt_Bremser]

It's my guess that MI uses the second variable as some kind of "help". But I'm no statistician, maybe @Rick_SAS can provide more insight.

[SAS_Rob]

MI is meant to impute based on a multivariate distribution and thus needs more than 1 variable.

[ThomasNord]

Are there any other SAS procedures made for the single variable imputation that you coud recommend using instead?

Thanks for the rply

Thomas

[Rick_SAS]

In the case of one variable, MI is similar to bootstrap resampling. For each imputed sample, you can replace each missing value with a random value from the nonzero values. For example, when forest=1, your data has

1 value of 0

1 value of 0.018552237

8 values of 0.070685835

It's not clear to me what you want to do with the forest=0 data, which doesn't have missing values. Copy it over to each imputed set?

Anyway, for the forest=1 data, you can write a program such as the following to replace missing values with a random observed value:

/* initial distribution of values */
proc freq data=NFI;
where forest=1;
tables A_forest / missprint;
run;

/* multiple imputations of the forest=1 data */
data Impute;
call streaminit(54321);
array Value[3] _temporary_ (0.070685835, 0.018552237, 0);
array Prob[3] _temporary_ (0.8, 0.1, 0.1);
set NFI(where=(Forest=1));
ObsNum = _N_;
do _Imputation_ = 1 to 5;
   if x = . then do;
      i = rand("Table", of Prob[*]);
      A_forest = Value[i];
   end;
   else ;
   output;
end;
run;

proc sort data=Impute;
   by _Imputation_ ObsNum;
run;

/* final distribution of values accross all imputed sets */
proc freq data=Impute;
   tables A_forest / missprint;
run;