R version 3.2.2 (2015-08-14) -- "Fire Safety"
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> x <- c(6678,6554,6513,6210,5928,6268,5582,5869,5764,6082,6062,6810,6727,6537,6175,6014,6109)
> par2 = '12'
> par1 = 'additive'
> par2 <- '12'
> par1 <- 'additive'
> #'GNU S' R Code compiled by R2WASP v. 1.2.327 ()
> #Author: root
> #To cite this work: Wessa P., (2013), Classical Decomposition (v1.0.4) in Free Statistics Software (v$_version), Office for Research Development and Education, URL http://www.wessa.net/rwasp_decompose.wasp/
> #Source of accompanying publication: Office for Research, Development, and Education
> #
> par2 <- as.numeric(par2)
> x <- ts(x,freq=par2)
> m <- decompose(x,type=par1)
Error in decompose(x, type = par1) : 
  time series has no or less than 2 periods
Execution halted