What may be considered a fair cost per unit of measure for a particular item differs greatly: First, with the point of view and breadth of horizon of the man stating such cost; and second, with the methods of letting contracts and accounting with which he may be familiar, as applied to such items of work. Because of the first, a fair average unit cost may mean one thing to a contractor, another to a division engineer, still another to a chief engineer, and a fourth to a manager or consulting engineer; and because of the second, the understanding of the term may differ among men of the same class. All of this quite aside from what may be termed the personal equation of the individual. Thus the subject of overhead charges can only be discussed profitably in the light of knowledge concerning what has already been included in fixing the unit prices used. For example, the element of hazard common to all construction, but differing in degree on different classes of work, may be included in the unit cost used, or it may be added as a percentage to resulting sums, but it cannot rightly be included twice. This is equally true of other elements of cost of a similar character. The foregoing is pertinent, for any valuation will probably be attacked in the Courts, and the unit values will be one of the most tempting points for assault, for the very reason that this wide difference of understanding in regard to cost, and particularly in regard to unit costs, exists. This same difference of understanding is usually the reason for the wide difference in unit costs testified to by able engineers and, consequently, for the distrust often felt for such testimony. The methods followed in taking expert testimony usually work to make "confusion worse confounded." The judge or layman, hearing two engineers testify to widely different unit prices as a fair average cost for certain work, forms a low opinion of their judgment, or worse, whereas the real difficulty may, and usually does, lie in a different understanding of the meaning of the term "cost," or "unit cost." To the speaker, this seems to be the weakest point in an admirable paper.