http://eprints.poltekkesjogja.ac.id/1362/4/BAB%20II.pdf
http://eprints.umpo.ac.id/5024/3/BAB%202.pdf
https://www.rsi.co.id/images/pmkp/ppk/ppk_tbparudewasa-2015.pdf
http://e-journal.sari-mutiara.ac.id/index.php/JAM/article/view/2215/1520
https://publikasiilmiah.ums.ac.id/bitstream/handle/11617/3409/13.%20ARIF.pdf;sequence=1