R help - Oct 2010 - How to find out the trend during a time period?

Hi all:
I have a dataset which contains two variables: Y and time
y<-c(228,189,232,198,252,315)
time<-2003:2008

How can I find out the trend(increase/decrease) of y along the time period?

If I use:
lm(y~time)

The "lm" command treats time as natural number,but not date.
So maybe "lm" isn't appropriate.

Then,which function could be used?

Thanks a lot for your help.

My best

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On 28.10.2010 06:50, Lao Meng wrote:
> Hi all:
> I have a dataset which contains two variables: Y and time
> y<-c(228,189,232,198,252,315)
> time<-2003:2008
>
> How can I find out the trend(increase/decrease) of y along the time period?
>
> If I use:
> lm(y~time)
>
> The "lm" command treats time as natural number,but not date.
What is "date"? WHere is the reproducible example?

Uwe Ligges


> So maybe "lm" isn't appropriate.
>
> Then,which function could be used?
>
> Thanks a lot for your help.
>
> My best
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
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> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

2010/10/28 Uwe Ligges <ligges at
statistik.tu-dortmund.de>:
>
>
> On 28.10.2010 06:50, Lao Meng wrote:
>>
>> Hi all:
>> I have a dataset which contains two variables: Y and time
>> y<-c(228,189,232,198,252,315)
>> time<-2003:2008
>>
>> How can I find out the trend(increase/decrease) of y along the time
>> period?
>>
>> If I use:
>> lm(y~time)
>>
>> The "lm" command treats time as natural number,but not date.
>
> What is "date"? WHere is the reproducible example?
The code the poster displayed is reproducible -- the problem is more
that its not entirely clear what is desired.  If the worry is that
there can be different numbers of days in different years due to leap
years then one could use the number of days between January 1st of
each year and January 1st of the first year like this (where y and
time are from the original post):
> d <- as.Date(paste(time, 1, 1, sep = "-"))
> d <- as.numeric(d - d[1])
> d
[1]    0  365  731 1096 1461 1826
> lm(y ~ d)
Call:
lm(formula = y ~ d)

Coefficients:
(Intercept)            d
  193.52454      0.04615



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