Tagliamonte, Sali A.
Analysing sociolinguistic variation
Cambridge University Press, 2006
 Buku Teks SO
Maguire, Warren
Analysing variation in english
Cambridge University Press, 2011
 eBooks
Enideg Nigussie, Ethiopia
Variation tolerant on-chip interconnects
[, Springer Science+Business Media], 2012
 eBooks
Wainwright, W. Howard
Variation and final account procedure
Hutchinson Technical Education, 1963
 Buku Teks SO
American government
by Susan Welch ... [and others]
Thomson, 2006
 Buku Teks SO