[R] Problems plotting and regression w.r.t. date data type on x axis

[David Winsemius]

On Feb 14, 2013, at 1:10 AM, Kategoricus wrote:

> Hello,
>
> probably a newbie question, but i didnt find any information on
> plotting/regressing w.r.t. a date data type. My trials were  
> unfruitful. Can
> anyone help ? Thanks in advance!
>
> Here is my interaction with R:
>
>
>> tabelle
> date number date2
> 1 2009-01-1 1673 2009-01-01
> 2 2009-12-1 2111 2009-12-01
> 3 2010-7-1 2487 2010-07-01
> 4 2013-2-1 4301 2013-02-01
>> regression.punkte<-lm(tabelle$number ~ tabelle$date2)
> Fehler in model.frame.default(formula = tabelle$number ~ tabelle 
> $date2, :
> ungültiger Typ (list) für die Variable 'tabelle$date2'

I'm guessing that you used as.POSIXlt to create that date2 variable.  
POSIXlt classed variables are lists and as such create all sorts of  
problems for functions that are expecting atomic vectors. If I'm  
right, you will ahve better chances of success by converting to  
POSIXct class.

 > tabelle <- read.table(text="date number date2
+ 1 2009-01-1 1673 2009-01-01
+ 2 2009-12-1 2111 2009-12-01
+ 3 2010-7-1 2487 2010-07-01
+ 4 2013-2-1 4301 2013-02-01", header=TRUE, stringsAsFactors=FALSE)
 > tabelle$date2 <- as.POSIXlt(tabelle$date2)
 > regression.punkte<-lm(tabelle$number ~ tabelle$date2)
Error in model.frame.default(formula = tabelle$number ~ tabelle 
$date2,  :
   invalid type (list) for variable 'tabelle$date2'

 > tabelle$date2 <- as.POSIXct(tabelle$date2)
 > regression.punkte<-lm(tabelle$number ~ tabelle$date2)
 > regression.punkte

Call:
lm(formula = tabelle$number ~ tabelle$date2)

Coefficients:
   (Intercept)  tabelle$date2
    -2.405e+04      2.082e-05


>> regression.punkte<-lm(tabelle$number ~ tabelle$date)
>> plot(tabelle$date2,tabelle$number, pch=19, xlab="date of retrieval",
>> ylab="number of animals") ***R DRAWS THIS***
>> regression.punkte
>
> Call:
> lm(formula = tabelle$number ~ tabelle$date)
>
> Coefficients:
> (Intercept) tabelle$date2009-12-1 tabelle$date2010-7-1
> 1673 438 814
> tabelle$date2013-2-1
> 2628
>
>> abline(regression.punkte, lwd=2) ***R DOES NOT DRAW LINE***
> Warnmeldung:
> In abline(regression.punkte, lwd = 2) :
> nutze nur die ersten beiden von 4 Regressionskoeffizienten
>> summary(regression.punkte)
>
> Call:
> lm(formula = tabelle$number ~ tabelle$date)
>
> Residuals:
> ALL 4 residuals are 0: no residual degrees of freedom!
>
> Coefficients:
> Estimate Std. Error t value Pr(>|t|)
> (Intercept) 1673 NA NA NA
> tabelle$date2009-12-1 438 NA NA NA
> tabelle$date2010-7-1 814 NA NA NA
> tabelle$date2013-2-1 2628 NA NA NA
>
> Residual standard error: NaN on 0 degrees of freedom
> Multiple R-squared: 1, Adjusted R-squared: NaN
> F-statistic: NaN on 3 and 0 DF, p-value: NA
>
>

David Winsemius, MD
Alameda, CA, USA