The second paragraph of this paper by Kouwenhoven et al. has a great summary:
Practically all O-type stars (Mason et al. 1998) and B/A-type stars (Shatsky & Tokovinin 2002; Kobulnicky & Fryer 2007; Kouwenhoven et al. 2007b) are found in binary or multiple systems. Abt & Levy (1976) report a multiplicity fraction of 55% among F3−G2 stars, and in their CORAVEL spectroscopic study of F7−G9 stars, Duquennoy & Mayor (1991) find a binary fraction of ∼60%. The binary fraction among M-type stars is 30−40% (Fischer & Marcy 1992; Leinert et al. 1997; Reid & Gizis 1997). For late M-type stars and brown dwarfs the binary fraction decreases to 10−30% (e.g., Gizis et al. 2003; Close et al. 2003; Bouy et al. 2003; Burgasser et al. 2003; Siegler et al. 2005; Ahmic et al. 2007; Maxted et al. 2008; Joergens 2008).
(Note those references can be found easily on the SAO/NASA ADS here.)
The rest of the paper describes some of the subtleties at work when astronomers combine modeling with observational data.