Dr. Shepard is testing a new drug to treat arthritis symptoms. He uses a sample group of 1,000 arthritis sufferers, and has them each take the drug when experiencing symptoms, such as swelling, pain, and redness of the joints. Of this sample group, 839 report that their arthritic symptoms seem to subside after taking the drug. From this data, Dr. Shepard concludes that the drug is effective. Is his conclusion scientifically valid?