A description of the candidate's activities at this club will probably provide a better evaluation of this candidate's public speaking skills. It may be easy to get a referee to give his or her opinion. The problem with an opinion, however, is that the opinion giver may have a very different frame of reference than the selection board. One must remember that the opinion giver may have observed the candidate in a context very different from that of the position being staffed. The opinion giver's standards may also differ from yours.
Extra care is necessary, therefore, in using opinions.
An attempt should be made to take into account the opinion giver's frame of reference. The weight to be given to unsupported or marginally supported opinions will depend on the pattern of other evidence. Behavioural descriptions are the most direct kind of evidence and are therefore to be preferred. Where possible, you should seek out behavioural descriptions. It may be possible to identify behaviours that correspond to increasing levels of a qualification, thus distinguishing the inadequate from the acceptable, the acceptable from the good, and so on.