[R] normalizing prices to a specific year - how to?

[David Winsemius]

On Jan 27, 2011, at 3:02 PM, Martin Hvidberg wrote:

> Dear list
>
> I have a large data frame with house prices. Prices have been  
> converted
> to price per m2, to be able to compare. Prices are spread over  
> almost 10
> years, each price is tagged with the year it relates to.
>
> I would like to 'normalize' all the prices to, eg. 2010 prices.
>
> The data frame looks like this
>
>> head(dafP)
>
> Y P
>
> 1 2004 11199.60
>
> 2 2005 15310.10
>
> 3 2005 19491.50
>
> 4 2005 18656.70
>
> 5 2005 16092.00
>
> 6 2005 9929.08
>
> Y is Year and P is price (in Danish Kroner) per m2
>
> I looked in some old scripts and have written this:
>
> dafP$Yav <- ave(dafP$P,dafP$Y,FUN=mean) # Yav. Prices Weight per Year,
> i.e. Yearly mean
>
> dafP$Pno <- dafP$P * (mean(dafP$P)/dafP$Yav) # Prize Normalized by  
> Year
>
> Can any one comment on this:
>
>    *
>
>      This code only normalizes so all year get the same mean. How  
> would
>      I go about 'normalizing' to a specific year, eg. 2010?

? tapply

Maybe something like:

mn2010 <- head(dafP[which(dafP$Y==2010), "Yav"], 1) # get just first  
2010 value
with(dafP,   tapply(P, Y, scale, center=mn2010) )

aggregate can be used with scale and may be better in some instances.  
Testing is not done here because you failed to provide a useful example.

-- 
David


>
> Best Regards
>
> Martin
>
>
> -- 
> */ Martin Hvidberg
>    Tanggårdsvej 22
>    2680 Solrød
>    tlf.: 35 83 01 91
>    email: Martin at Hvidberg.net /*
>
>
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David Winsemius, MD
West Hartford, CT