[R] subsamples and regressions for 100 times

[Michael Dewey]

Comment inline

On 17/02/2015 12:40, Angela Smith wrote:
>
>
> Hi R user,
> I'm new to R so
> my problem is probably pretty simple but I'm stuck:
>
>
>
> my data is consist of 2 variables: co2, temp and one
> treatment (l_group). The sample size is different among the treatments. so
>
> that, I wanted to make equal sample size among three groups (A,B and C) of the
> treatment.
>

Not sure whether that is necessary for regression but you did not tell 
us why you want to do that.

> For this one, I used subsamples technique. Using
> subsample, each time the data are different among the three groups of the
> treatment.
>
> so that I want to run regression (co2~temp) for a 100
> subsamples for each group of treatment (100 times subsample).
>

The usual way to do this is to store the subsamples in a list and then 
write a function and use lapply, say to store your models. You then have 
another list to which you can then apply the extractor function of your 
choice.


> it means that I will have 100 regression equations.  Later, I want to compare the slope of the
> regression among the three groups. is there simple way to make a loop so that I
> can compare it?
>
> Thanks in advance!
>
>
>
> Angela
>
> ================
> Here is the example:
>
> dat<-structure(list(co2 = c(0.15, 0.148, 0.125, 0.145, 0.138, 0.23,
> 0.26, 0.35, 0.41, 0.45, 0.39, 0.42, 0.4, 0.43, 0.26, 0.3, 0.34,
> 0.141, 0.145, 0.153, 0.151, 0.128, 0.23, 0.26), temp = c(0.0119,
> 0.0122, 0.0089, 0.0115, 0.0101, 0.055, 0.097, 0.22, 0.339, 0.397,
> 0.257, 0.434, 0.318, 0.395, 0.087, 0.13, 0.154, 0.0107, 0.0112,
> 0.0119, 0.012, 0.0092, 0.055, 0.089), L_group = structure(c(1L,
> 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 3L, 3L, 3L,
> 3L, 3L, 3L, 3L, 3L, 3L, 3L), .Label = c("A", "B", "C"), class = "factor")), .Names = c("co2",
> "temp", "L_group"), class = "data.frame", row.names = c(NA, -24L
> ))
>
> head(dat)
> library(sampling)
>
> # strata.sampling -----
> strata.sampling <- function(data, group,size, method = NULL) {
>   require(sampling)
>    if (is.null(method)) method <- "srswor"
>    temp <- data[order(data[[group]]), ]
>    ifelse(length(size)> 1,
>           size <- size,
>           ifelse(size < 1,
>                  size <- round(table(temp[group]) * size),
>                  size <- rep(size, times=length(table(temp[group])))))
>    strat = strata(temp, stratanames = names(temp[group]),
>                   size = size, method = method)
>    getdata(temp, strat)
> }
>
> #--------------------------------------------------
> sub_dat <- strata.sampling(dat, 'L_group', 4)#
> Lmodel_subdata1<-lm(co2~temp, data=subdat)
> Lmodel_subdata1#coef
>
> sub_dat2 <- strata.sampling(dat, 'L_group', 4)#
> Lmodel_subdata2<-lm(co2~temp, data=subdat2)
> Lmodel_subdata2#coef
>
> and so on.....[for 100 times)
>
> Table<-rbind(Lmodel_subdata1$coef, Lmodel_subdata1$coef, ....)
>
>
>   		 	   		
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-- 
Michael
http://www.dewey.myzen.co.uk