How does a law firm determine which medical malpractice cases to accept?

A law firm's decision to accept a case is based upon several factors: (1) is the negligence of the doctor, nurse, or hospital clear, i.e. can we find reputable experts to criticize the care? (2) is the medical error reflected in the medical record or will it have to be establsihed by disputed testimony (doctors win most swearing contests)? (3) is there anything in the records that suggests that the patient did not follow doctrs' orders? (4) did the medical error really change the outcome, i.e. would the patient not have suffered the injury complained of if proper care was given? (5) how serious is the injury, i.e. is the injury permanent, are there large medical expenses and/or a big income loss?