Editing Small-world experiment (section)

===Criticisms===
There are a number of methodological criticisms of the small-world experiment, which suggest that the average path length might actually be smaller or larger than Milgram expected. Four such criticisms are summarized here:

# Judith Kleinfeld argues<ref>{{cite journal |last1=Kleinfeld |first1=Judith |date=March 2002|title=Six Degrees: Urban Myth? |journal=Psychology Today |publisher=Sussex Publishers, LLC |url=http://www.psychologytoday.com/articles/200203/six-degrees-urban-myth |access-date=June 15, 2011}}</ref> that Milgram's study suffers from selection and non-response bias due to the way participants were recruited and high non-completion rates. First, the "starters" were not chosen at random, as they were recruited through an advertisement that specifically sought people who considered themselves well-connected. Another problem has to do with the attrition rate. If one assumes a constant portion of non-response for each person in the chain, longer chains will be under-represented because it is more likely that they will encounter an unwilling participant. Hence, Milgram's experiment should underestimate the true average path length. Several methods have been suggested to correct these estimates; one uses a variant of [[survival analysis]] in order to account for the length information of interrupted chains, and thus reduce the bias in the estimation of average degrees of separation.<ref name = "Schnettler2009">{{cite journal | doi=10.1016/j.socnet.2008.12.005 | title=A small world on feet of clay? A comparison of empirical small-world studies against best-practice criteria | date=2009 | last1=Schnettler | first1=Sebastian | journal=Social Networks | volume=31 | issue=3 | pages=179–189 }}</ref>
# One of the key features of Milgram's methodology is that participants are asked to choose the person they know who is most likely to know the target individual. But in many cases, the participant may be unsure which of their friends is the most likely to know the target. Thus, since the participants of the Milgram experiment do not have a topological map of the social network, they might actually be sending the package further away from the target rather than sending it along the [[Shortest path problem|shortest path]]. This is very likely to increase route length, overestimating the average number of ties needed to connect two random people. An omniscient path-planner, having access to the complete social graph of the country, would be able to choose a shortest path that is, in general, shorter than the path produced by a [[greedy algorithm]] that makes local decisions only.
# A description of heterogeneous social networks still remains an open question. Though much research was not done for a number of years, in 1998 [[Duncan Watts]] and [[Steven Strogatz]] published a breakthrough paper in the journal ''Nature.'' Mark Buchanan said, "Their paper touched off a storm of further work across many fields of science" (''Nexus'', p60, 2002). See Watts' book on the topic: ''[[Six Degrees: The Science of a Connected Age]]''.
# Some communities, such as the [[Sentinelese people|Sentinelese]], are completely isolated, disrupting the otherwise global chains. Once these people are discovered, they remain more "distant" from the vast majority of the world, as they have few economic, familial, or social contacts with the world at large; before they are discovered, they are not within any degree of separation from the rest of the population. However, these populations are invariably tiny, rendering them of low statistical relevance.

In addition to these methodological criticisms, conceptual issues are debated. One regards the social relevance of indirect contact chains of different degrees of separation. Much formal and empirical work focuses on diffusion processes, but the literature on the small-world problem also often illustrates the relevance of the research using an example (similar to Milgram's experiment) of a targeted search in which a starting person tries to obtain some kind of resource (e.g., information) from a target person, using a number of intermediaries to reach that target person. However, there is little empirical research showing that indirect channels with a length of about six degrees of separation are actually used for such directed search, or that such search processes are more efficient compared to other means (e.g., finding information in a directory).<ref name = "Schnettler2009b">{{cite journal | doi=10.1016/j.socnet.2008.12.004 | title=A structured overview of 50 years of small-world research | date=2009 | last1=Schnettler | first1=Sebastian | journal=Social Networks | volume=31 | issue=3 | pages=165–178 }}</ref>