Re: [R] proportional sampling On 27-Apr-07 12:15:29, tibi.codilean@ges.gla.ac.uk wrote: > Dear All, >  > I wonder if you could help me. >  > I have a table with a series of sites that I would like to > sample from. > The table has 5 columns: >  > ID > X Coordinate > Y Coordinate > Value > Factor >  >  > The conditions are that each site can be selected more than > once and the probability of it being selected (or sampled) > is proportional to a factor located in column 'Factor' >  > I am novice in terms of R, and am not entirely sure how to > do the proportional sampling. >  > Any help would be appreciated > Thanks > Tibi Since you want each site to be able to appear more than once in the sample, there should be no problems in using sample():   ID.sample <- sample(ID, N, replace=TRUE, prob=Factor) where N is the sample size you want. (You do not need to make Factor sum to 1: sample() looks after that). Or, if you want an index which you can use to identify whole rows (especially if, e.g., values of ID are repeated in the table):   ix <- sample((1:R), N, replace=TRUE, prob=Factor) where R is the number of rows in the table. Then your sample is the subset   Table[ix,] of rows of the table (where "Table" stands for the name of your table). There are more complicated issues which can arise if you are sampling without replacement with probability proportional to some variable. Have a look at the packages 'pps' and 'sampfling' for an indication of methods. Hoping this helps, Ted. -------------------------------------------------------------------- E-Mail: (Ted Harding)  Fax-to-email: +44 (0)870 094 0861 Date: 27-Apr-07                                       Time: 14:07:03 ------------------------------ XFMail ------------------------------ ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.