Tagliamonte, Sali A
Analysing sociolinguistic variation
Cambridge University Press , 2006
 Buku Teks
Maguire, Warren, editor
Analysing variation in english
Cambridge University Press, 2011
 eBooks
Enideg Nigussie, Ethiopia
Variation tolerant on-chip interconnects
[, Springer Science+Business Media], 2012
 eBooks
Patterson, Thomas E.
The American democracy
McGraw-Hill, 203
 Buku Teks
Spring, Joel
American education
McGraw-Hill, 2008
 Buku Teks