Below is a list of studies that have been conducted on the various NFP methods.
Billings Method
https://www.ncbi.nlm.nih.gov/pubmed/1755470
https://www.contraceptionjournal.org/article/0010-7824(88)90154-0/fulltext
Creighton Model FertilityCare System
https://triggered.clockss.org/ServeContent?rft_id=info:doi/10.1001/archfami.8.5.391
https://www.ncbi.nlm.nih.gov/pubmed/8057183 - written by Marquette Founder
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6027022/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5596066/
https://www.frontiersin.org/articles/10.3389/fmed.2017.00147/full
Marquette Method
https://pubmed.ncbi.nlm.nih.gov/27926599/
https://www.ncbi.nlm.nih.gov/pubmed/27926599
https://www.ncbi.nlm.nih.gov/pubmed/23288279
https://www.ncbi.nlm.nih.gov/pubmed/28586636
https://www.jognn.org/article/S0884-2175(15)30021-6/abstract
https://www.marquette.edu/nursing/directory/richard-fehring.php
Sympto-Thermal Method
See side bar for additional resources
https://www.ncbi.nlm.nih.gov/pubmed/10794048
https://ccli.org/docs/frank-herrmann-2007-study.pdf or https://academic.oup.com/humrep/article/22/5/1310/2914315
https://www.jabfm.org/content/22/2/147#ref-49
https://academic.oup.com/humrep/article/18/9/1959/708200
https://academic.oup.com/humrep/article/22/5/1310/2914315
General NFP Research
https://www.aafp.org/afp/2012/1115/p924.html#afp20121115p924-b29