If you would prefer to fill in any of these questionnaires offline you can download a pdf by clicking on the link.
The range of scores on the PHQ-15 is 0-30.
Kocalevent et al. (2013) state that evidence supports the PHQ-15 as a valid and reliable measure of somatisation symptoms. The American Psychiatric Association states the following levels of somatic symptom severity: 0-4 "minimal; 5-9 "low"; 10-14 "medium"; and 15-30 "high".
References
American Psychiatric Association (n.d.). LEVEL 2 - Somatic symptom - Adult patient. https://www.psychiatry.org/File%20Library/Psychiatrists/Practice/DSM/APA_DSM5_Level-2-Somatic-Symptom-Adult.pdf
Kocalevent, RD., Hinz, A. & Brähler, E. (2013). Standardization of a screening instrument (PHQ-15) for somatization syndromes in the general population. BMC Psychiatry 13, 91. https://doi.org/10.1186/1471-244X-13-91
Kroenke, K., Spitzer, R. L., & Williams, J. B. (2002). The PHQ-15: validity of a new measure for evaluating the severity of somatic symptoms. Psychosomatic Medicine, 64(2), 258-266.
The range of scores for the IBS-SSS is 0-500. Francis et al. (1997) report that a score of 75-175 indicates "mild" severity, 175-300 "moderate" severity, and 300 plus "severe" severity.
Reference
Francis, C. Y., Morris, J., & Whorwell, P. J. (1997). The irritable bowel severity scoring system: A simple method of monitoring irritable bowel syndrome and its progress. Alimentary Pharmacology & Therapeutics, 11(2), 395-402.