6. Another important point is the determination of the density of population, that is, the number of individuals per unit of surface area. Most commonly for larger areas such as states and countries the population is divided by the total area in miles or acres (Fig. 126). For comparative studies in an urban population it is perhaps better to analyze data according to ward populations rather than by density per acre.
In considering population fluctuations within civil areas in successive years it is important to aseerta;n if any changes have been made in the geographical boundaries of the area under con sideration. Large increases may be due to the annexation of territory with a considerable population, and sudden increases thus appear.
6. Value of Vital any consideration of various facts concerning a large body of individuals, such as the popula tion of a city, a state or a nation as regards births, deaths or individuals, it is impossible to consider the individual by him self, but rather all must be considered in the aggregate if any clear conception is to be gained of the influence of economic or morbific factors. Considered as individuals we would lose sight of the mass. As a consequence, it is necessary to employ statistical methods to avoid confusion and to enable the trend of events to be clearly discerned.
In any employment of statistical methods it is of course necessary to express facts by figures, and as expressed by Whip ple: " When figures are used to express facts, and when logical processes are applied to figures, divorced in the mind from the facts for which they stand, it is easy for fallacies to creep in without being recognized; it is easy to compare things that ought not be compared; to generalize from inadequate data and so commit all sorts of illogical errors. Thus the unscrupu
lous may fool the unwary and the innocent fool themselves. Hence to use statistics properly one must be able not only to visualize the facts but to think logically." Quoting Whipple further: "Vital statistics are useful for many purposes. To the his torian they show the nation's growth and mark the flood and ebb of physical life; to the economist they indicate the number and distribution of the producers and the consumers of wealth; to the sanitarian they measure the people's health and reflect the hygienic conditions of their environment; to the sociologist they show many things relating to human beings in their rela tions one with another." "Vital statistics are not to be collected and used as mere records of past events; an even more important use is that of prophesying the future. The health officer should study them as as received and not wait until some convenient day when other work is slack and then merely tabulate them and make averages for formal reports and permanent records. Vital statistics, especially those of morbidity, should be studied in the making and just as the meteorologist reads his instruments daily in order to forecast the weather and give warnings of the coming hurricane, so the efficient health officer will daily study the reports of new cases of disease in order that he may be fore warned of an impending epidemic and take measures to check its ravages."