Citation patterns are also known to be affected by unethical behavior of both the authors and journal staff. Such behavior is called impact factor boosting, and was reported to involve even the top-tier journals. Specifically the high-ranking journals of medical science, including the Lancet, JAMA and New England Journal of Medicine, are thought to be associated with such behavior, with up to 30% of citations to these journals being generated by commissioned opinion articles.  On the other hand, the phenomenon of citation cartels is rising. Citation cartels are defined as groups of authors that cite each other disproportionately more than they do other groups of authors who work on the same subject. 
A friend or colleague usually gives this type of reference. Accordingly, the reference is highly unlikely to say anything bad about the prospective tenant. It can however offer a slightly increased assurance as to their character. This reference is generally of little importance unless the referee is an individual with a high-ranking position of authority. If you obtain a phone number and should the tenant disappear you can make discreet enquiries, this is better than embarking on a lengthy abandonment or eviction process, worse still defending an unlawful eviction claim.