[R] Error of Stepwise Regression with number of rows in use has changed: remove missing values?

[Peter Ehlers]

On 2010-02-16 1:24, Kum-Hoe Hwang wrote:
> Howdy, R Grues
>
> I have enjoyed R, but I cannot solve one problem easily. Please help my problem.
> When I tried the R script, I got the following Error. This error
> results from input data file exported through a Excel spreadsheet
> software.
>
>   Error in step(lm(pop.rate ~ as.numeric(year) + as.factor(policy) +
> as.numeric(nation.grant) +  :
>    number of rows in use has changed: remove missing values?
>
> Could you direct me to solve the Error?
> Thanks in advance,

This is a common situation when you use step() on data where
the predictors have missing values.

A case (row) is included in the model only if all the
predictors for that model are non-missing for the case.

As you vary which predictors are to be in the model, the
included cases will vary, resulting in models based on
different data. (Think of your cases as subjects; you want
all your models to be based on the same set of subjects.)

Finally: (Re-)read the help page and note the 'warning'.

  -Peter Ehlers

>
>
>> ############### outputs from R console ###############
>> pop<- step(
> +             lm(pop.rate ~ as.numeric(year) + as.factor(policy) +
> as.numeric(nation.grant)
> +                + as.numeric(do.grant) + as.numeric(city.grant) +
> as.numeric(DMZ.dist) + as.numeric(Seoul.dist), data=borderI.data,
> na.action = na.omit)
> +             )
> Start:  AIC=494.27
> pop.rate ~ as.numeric(year) + as.factor(policy) + as.numeric(nation.grant) +
>      as.numeric(do.grant) + as.numeric(city.grant) + as.numeric(DMZ.dist) +
>      as.numeric(Seoul.dist)
>                             Df Sum of Sq    RSS    AIC
> - as.numeric(do.grant)      1      0.71 6622.9 492.28
> - as.factor(policy)         1      1.21 6623.4 492.29
> - as.numeric(DMZ.dist)      1      1.91 6624.1 492.30
> - as.numeric(city.grant)    1      5.07 6627.3 492.36
> - as.numeric(nation.grant)  1     11.51 6633.7 492.47
> - as.numeric(year)          1     29.58 6651.8 492.80
> <none>                                    6622.2 494.27
> - as.numeric(Seoul.dist)    1    673.22 7295.4 503.79
> Step:  AIC=492.28
> pop.rate ~ as.numeric(year) + as.factor(policy) + as.numeric(nation.grant) +
>      as.numeric(city.grant) + as.numeric(DMZ.dist) + as.numeric(Seoul.dist)
>                             Df Sum of Sq    RSS    AIC
> - as.factor(policy)         1      1.99 6624.9 490.32
> - as.numeric(DMZ.dist)      1      2.09 6625.0 490.32
> - as.numeric(city.grant)    1      7.18 6630.1 490.41
> - as.numeric(nation.grant)  1     20.08 6643.0 490.64
> - as.numeric(year)          1     28.89 6651.8 490.80
> <none>                                    6622.9 492.28
> - as.numeric(Seoul.dist)    1    697.46 7320.4 502.20
> Step:  AIC=490.32
> pop.rate ~ as.numeric(year) + as.numeric(nation.grant) +
> as.numeric(city.grant) +
>      as.numeric(DMZ.dist) + as.numeric(Seoul.dist)
>                             Df Sum of Sq    RSS    AIC
> - as.numeric(DMZ.dist)      1      2.08 6627.0 488.35
> - as.numeric(city.grant)    1     10.65 6635.6 488.51
> - as.numeric(nation.grant)  1     31.30 6656.2 488.88
> - as.numeric(year)          1     31.44 6656.4 488.88
> <none>                                    6624.9 490.32
> - as.numeric(Seoul.dist)    1    732.88 7357.8 500.80
> Step:  AIC=488.35
> pop.rate ~ as.numeric(year) + as.numeric(nation.grant) +
> as.numeric(city.grant) +
>      as.numeric(Seoul.dist)
>                             Df Sum of Sq    RSS    AIC
> - as.numeric(city.grant)    1      9.86 6636.9 486.53
> - as.numeric(year)          1     31.42 6658.4 486.92
> - as.numeric(nation.grant)  1     33.33 6660.3 486.95
> <none>                                    6627.0 488.35
> - as.numeric(Seoul.dist)    1    754.40 7381.4 499.18
>
> Error in step(lm(pop.rate ~ as.numeric(year) + as.factor(policy) +
> as.numeric(nation.grant) +  :
> -------------------------------------------------------------------------------------------------------------------------------------------
>    number of rows in use has changed: remove missing values?
> ------------------------------------------------------------------------------------------
>
>
>
>
> --
> Kum-Hoe Hwang, Ph.D.
>
> Phone : 82-31-250-3516
> Email : phdhwang at gmail.com
>

-- 
Peter Ehlers
University of Calgary