There is a technical use of the term "clinical impression" which is what I think they are using here. It does not mean a simple diagnosis, but rather a quantified assessment of the effectiveness of a type of practice or treatment after several trials with one patient, or more likely across serveral patients. It is assessment of the treatments with many patients that gives you the global clinical impression.

HERE ARE 2 EXAMPLES FROM US GOVT WEBSITES

The first about clinical trials of a new cancer treatement:

"The clinician will assign a clinical impression (which may be a final definitive diagnosis, or a suspected diagnosis, or "unable to determine"). Follow-up contacts and assessments are recorded as they occur. Each time the patient returns for an assessment, the clinical impression can be updated, until a definitive diagnosis is made. A research coordinator will be tasked to monitor the Quantbook to provide quality control that the clinical data entered is complete and correct."

http://clinicaltrials.gov/ct2/show/NCT00194207

About animal drug testing:

Entrance Criteria: Treatment was initiated once at least 10% of pigs in each barn manifested either Grade 3 diarrhea scores or Grade 2 clinical impression scores.
http://www.fda.gov/downloads/AnimalVeterinary/Products/Appro...

AND THESE ON GLOBAL CLINICAL IMPRESSION

Methods In a drug surveillance study of the antidepressant citalopram, the scores of patients' and clinicians' assessment of global change (Global Clinical Impression Scale) were compared, grouping those patients that showed any kind of improvement in each follow-up assessment
http://priory.com/psych/cormina.htm

At every visit (weekly), each patient received a diary in which they recorded any panic attack that occurred between the visits. The scales and measurements used for evaluating the clinical response were: 1) changes in the number of panic attacks in comparison with the baseline period; 2) the Global Clinical Impression scale and Global Patient's Impression scale scores for panic disorder;
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516...

AND related to acupuncture:

Using a 7-point Likert scale of global clinical impression as an outcome measure they estimated that about 50% of all patients showed relevant improvements, a figure that matches our estimates. Several other investigations from different countries in Europe or America report similar health effects in various diseases within the first year after homeopathic treatment
http://www.biomedcentral.com/1471-2458/8/413