5. He has a less number of patients, and a smaller amount of sickness, in the same number of families. In order to discover this difference between skilful and unskilful practice, the observation must be extended over some length of time, and quite a large number of families. That these points of difference between the results of good, and those of bad practice, may be appreciated with any correctness, two things are necessary. First. We must have a sufficient quantity of evidence. A few facts will not avail in deciding such points—they will only lead to erroneous conclusions. Comparison is to be made, it is true, between individual cases, but there must be many of them in order to secure the avoidance of error. The second requisite is the capability of observing correctly. I have altogether failed of obtaining one of my principal objects in this and the previous chapters, if the reader’s mind is not strongly impressed with the great liability to error, which attends careless and unskilful observation in medicine. There is no subject, in the wide range of human knowledge, the investigation of which requires more care and skill than this does. Now it is obvious, that the community in general are very deficient in these two requisites for a proper appreciation of the comparative results of practice. Most men have a very narrow range of facts, upon which they can found such an appreciation. Their observation of sickness extends little beyond their immediate family circle; for what they see of disease any where else is not like watching over it, and what they hear, as you have already seen, is not to be relied upon. There is much which is styled fact that is not so—it is either mis-statement, or the result of hasty and superficial observation. The actual knowledge which any non-professional observer obtains of disease, by any observation of his own, to which he gives any fair amount of attention is very narrow. This assertion may be offensive to many, who are accustomed to utter their opinions, in regard to the results of medical practice, with almost the authority of an oracle. But it is nevertheless true. But, the reader will say, if it be so difficult, and almost impossible, for me to discriminate between good and bad practice, by my own observation of their results, what shall I do? How shall I judge of the different modes of practice, and of the skill of different physicians? In answer to this enquiry, I remark, that if the reader is really convinced by what I have said, that it is almost impossible for him to judge of practice by the results which come within the compass of his observation, then it is plain that he must give up, for the most part, this source of evidence as a deceptive one, and rely upon other means for arriving at correct conclusions on these points. What these means are I shall point out in the chapter on Popular Estimates of Physicians.