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. 2009 Sep 7;4(9):e6777. doi: 10.1371/journal.pone.0006777

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Poon et al.

This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are properly credited.

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Strings generated by an SCFG are depicted on the left-hand side. is the start symbol from which all strings are derived, corresponding to the initiation of an RDS study; and are non-terminal symbols for different types of respondents whose recruitment outcomes are unresolved; and and are terminal symbols for respondents whose recruitment outcomes are resolved. A successful recruitment is indicated by a substring enclosed in parentheses to the immediate right of the originating terminal symbol. To represent the termination of an RDS study, the remaining non-terminal symbols are rewritten as lower-case terminal symbols, e.g., .

Inline graphic — Strings generated by an SCFG are depicted on the left-hand side. is the start symbol from which all strings are derived, corresponding to the initiation of an RDS study; and are non-terminal symbols for different types of respondents whose recruitment outcomes are unresolved; and and are terminal symbols for respondents whose recruitment outcomes are resolved. A successful recruitment is indicated by a substring enclosed in parentheses to the immediate right of the originating terminal symbol. To represent the termination of an RDS study, the remaining non-terminal symbols are rewritten as lower-case terminal symbols, e.g., .