rating systems differ accross firms but you ought to treat all broker research with a degree of caution as much of it is biased and an additional proportion of broker research is either flawed or inaccurate.
also bear in mind that there are not nearly enough sell recommendations out there either. brokers need access to companies in order to produce the research in the first place and that often encourages analysts to generate more optimistic earnings reports lest contact with the company is curtailed. moreover, the corporate advisory business of any broker is normally a lot more lucrative than the old fashioned broking activities and this tends to put further pressure on analysts to 'acceptable' research.