2
$\begingroup$

I estimate a model with linear regression:

model <- lm(orders ~ 
          day +
         holiday_dummies 
        ,data=training_df, na.action = na.exclude)

summary(model)

my response variable is zero somewhere but positive - in fact:

summary(df$orders)

   Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
  0.000   0.000   0.000   3.174   4.000  67.000 

When I try to estimate the boxcox like:

bc<-boxcox(model,plotit = T) 

I get:

 Error in boxcox.default(model, plotit = T): response variable must be positive

I shouldnt be a problem to have zero values right?

Hints are welcome!

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6
  • $\begingroup$ Not really. The box-cox transformation includes the log transformation as a particular case. See, e.g., here r-bloggers.com/on-box-cox-transform-in-regression-models. $\endgroup$
    – utobi
    Nov 28, 2016 at 14:34
  • $\begingroup$ that would mean that both lambdas are zero. is there a way to fix one of them and just keep (log + c) and estimate just one? @utobi $\endgroup$ Nov 28, 2016 at 14:38
  • 2
    $\begingroup$ There is only one lambda in box-cox transformation. A quick fix would be to add a small positive value to all of your observations prior to the box-cox transformation. Otherwise a zero-inflated model might be more appropriate. $\endgroup$
    – utobi
    Nov 28, 2016 at 15:00
  • 1
    $\begingroup$ As @utobi suggests since more than half your observations are zero any transformation is going to have a spike at the value which is the transformed zero so choosing a model which explicitly allows for clumping at zero looks a much better bet. $\endgroup$
    – mdewey
    Nov 28, 2016 at 15:05
  • 1
    $\begingroup$ Perhaps this article from JSS will help. $\endgroup$
    – mdewey
    Nov 28, 2016 at 15:15

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