30.  Statistics.  Statistics, which are collections of figures, are notoriously treacherous.  On many important subjects, such, for example, as the practical effect of the elective system, it is impossible to get them; and on many other subjects, such as the effects of a protective tariff, they must be had in so enormous masses, if they are to be trusted at all, that only profound students can handle them.  Where the facts are complicated, and interests are tangled, moreover, many sets of figures may enter into the question, as notably in the case of a tariff; so clearly is this difficulty now recognized that Congress has authorized a tariff board made up of distinguished students of economics and men of long experience in dealing with tariff matters to collect and study the facts and make recommendations based on them.  Similarly, with the investigation into the liquor question made fifteen years ago by the Committee of Fifty:  the whole question had been so tangled by assertion and counter-assertion that it became desirable to have an investigation into the facts by men of recognized ability and impartiality.[20]

In general, to use statistics safely you need a wide acquaintance with a subject, especially where the question is in any way mixed up with men’s feelings, whether through politics or not.  All the statistics we have make dead against great armaments and preparation for war; yet while human nature is what it is, necessary prudence seems to require every nation of any size to have them.  A very little human nature will upset a very great body of statistics.  Furthermore, in most human affairs results are produced by a multiplicity of causes; and though statistics may throw light on three quarters of all the causes that are potent in any given case, yet the other quarter which are irreducible to definite statement may wholly alter the result.  If you are using statistics in your argument, therefore, as evidence of some large and complex fact, you should usually justify them to some extent by showing that there are no counteracting forces which they do not cover.

With this precaution, however, statistics are the foundation of most arguments on large questions.  If you were arguing in favor of the purchase of local waterworks, you would present figures showing the number of houses using the public water supply, the rates paid, the profits of the company, the exact points at which public control could work economies.  If you were arguing for a rule that no man shall play on a university team until he has been registered a year at the university, you would need statistics to show how many men would be affected by the rule.  If you were arguing for a single session at a school instead of two, you would show exactly how many students in the school live more than a mile away from the building.  In every case where statistics can be presented in such a way as to make clear that they fairly cover the ground, they are most valuable evidence.