"Um" - is a sound that BBC radio producers often have to remove when they edit interviews. But what are we doing when we say "um" and "ah", and what effect do these sounds have on people listening to us? 
Michael RosenRadio 4's 'Word of Mouth' host Michel Rosen started his investigation into 'speech disfluency' by speaking to linguist Martin Corley. When Rosen asked if there were any benefits of the pesky "um", Corely was keen to point out its usefulness.
Word of Mouth program