If your first question can be rephrased as 'How can the frequency of words in published articles on sociolinguistics be measured?', the first step would be to create a corpus of articles on sociolinguistics and use some kind of corpus tool to count the words.

I think you will find an answer to your second question in: Bybee, Joan. (2007). Frequency of use and the organisation of language. Oxford: Oxford University press.

Hello Sadiq

these books and article include the relevant information  :

https://books.google.co.th/books?id=gsPb_GHZ-JIC&pg=PA24&lpg=PA24&dq=frequency+of+words+measured+in+sociolinguistic+studies&source=bl&ots=8_sh3Td4B4&sig=M8RVSQfneTzlL_FYM8Qegdn9ZSQ&hl=en&sa=X&ei=xA0SVafDOsTdmAW-kIDICw&ved=0CCUQ6AEwAw#v=onepage&q=frequency%20of%20words%20measured%20in%20sociolinguistic%20studies&f=false

http://repository.upenn.edu/cgi/viewcontent.cgi?article=1001&context=pwpl

Pragmatics_and_Variationist_Sociolinguistics[1]-3.pdf

https://books.google.co.th/books?id=grSuAwAAQBAJ&pg=PT202&lpg=PT202&dq=frequency+of+words+measured+in+sociolinguistic+studies&source=bl&ots=VTjNpHPoIn&sig=yZji6rzporV-jhdeIfHPLVlOeQI&hl=en&sa=X&ei=oQ4SVZ_MAZXn8AXk64GYDw&ved=0CCAQ6AEwAjgK#v=onepage&q=frequency%20of%20words%20measured%20in%20sociolinguistic%20studies&f=false

http://www.ling.upenn.edu/~wlabov/Papers/LSF.LVC.pdf

Have you used a Concordancing tool? I suggest AntConc: 

http://www.laurenceanthony.net/software.html

Very useful to count word frequencies, easy to operate and free. All you need is a corpus in .txt format. Once you have downloaded AntConc into your computer, you upload the corpus and it's ready to work.

Best regards.

Either in a large general reference corpus, such as the BNC or using a concordancer (AntConc, Wordsmith) should you go for measuring frequencies in a DIY corpus. 

HTH

If you create your study corpus and you also have a reference corpus, you can establish norms of occurrence and then measure distance from and proximity to those norms. Raw frequency counts can be deceptive if you don't really have a norm for comparison. Another option if you don't have a reference corpus or corpora (of similar genre) is to establish ratios of occurrence in addition to raw frequencies, which will give you a more comprehensive picture of patterns of occurrence. Good luck!

I agree with Martin. It depends on the text or groups of texts that use are using. All the same you have to create a study corpus and use corpora to determine the pattern of occurrence. The books suggested by Rahimi should assist you with the general conceptualization of the process.