The proportional similarity index (PSI) or Czekanowski index is an objective and simple estimate of the area of intersection between two frequency distributions (Rosef, Kapperud et al. 1985). The PSI estimates the similarity between the frequency distributions of i.e. bacterial sub types between different reservoirs. It is calculated by:
where pi and qi represent the proportion of strains belonging to type i out of all strains typed from species P and Q (Feinsinger, Spears et al. 1981; Rosef, Kapperud et al. 1985). Click on the below animation for a visualisation of the approach. The values for PS range from 1 for the highest possible similarity to 0 for distribution with no common types. Bootstrap confidence intervals for this measure can be estimated based on the approach applied by Garrett et al (Garrett, Devane et al. 2007). This technique has also recently applied to support source attribution studies of human campylobacteriosis (Mullner et al. 2009; Mullner et al. 2010).
The occurrence of non-typable strains in a dataset requires special attention when applying this method (Rosef, Kapperud et al. 1985; Garrett, Devane et al. 2007). The assumption made by this method is that epidemiological affinity between species is proportional to the similarity between the type distributions of the species being compared. This may be incorrect since many animal isolates may not be pathogenic, even if they are identical as determined by the typing method used. If a source also contains a high proportion of non-pathogenic strains its importance as contributor to human cases may be masked (Garrett, Devane et al. 2007). In addition some of the human cases may have originated from non included sources, i.e. by travel (Rosef, Kapperud et al. 1985).
Feinsinger, P., Spears, E. E., & Poole, R. W. (1981). A Simple Measure of Niche Breadth. Ecology, 62(1), 27-32.
Garrett, N., Devane, M. L., Hudson, J. A., Nicol, C., Ball, A., Klena, J. D., et al. (2007). Statistical comparison of Campylobacter jejuni subtypes from human cases and environmental sources. Journal of Applied Microbiology, 103(6), 2113-2121.
Mullner, P., Spencer, S. E. F., Wilson, D., Jones, G., Noble, A. D., Midwinter, A. C., et al. (2009). Assigning the source of human campylobacteriosis in New Zealand: A comparative genetic and epidemiological approach. Infection, Genetics and Evolution, 9, 1311-1319
Müllner, P., Collins-Emerson, J., Midwinter, A., Carter, P., Spencer S., Van derLogt, P., et al. (2010). Molecular epidemiology of Campylobacter jejuni in a geographically isolated country with a uniquely structured poultry industry. Applied and Environmental Microbiology, 76(7), 2145-2154.
Rosef, O., Kapperud, G., Lauwers, S., & Gondrosen, B. (1985). Serotyping of Campylobacter-jejuni, Campylobacter coli and Campylobacter-lardis from domestic and wild animals. Applied and Environmental Microbiology, 49(6), 1507-1510.