When certain series of observations showing frequency data are plotted on arithmetic probabiHty paper, the points do not fall in a straight line, but in a curve. Plotted on probability paper with a logarithmic scale as the ordinate, the points may fall approximately in a straight line or a gentle curve. In order to benefit from the use of probability paper, it is not necessary that the plotted points fall exactly in a straight line. If the curve is so gentle and uniform that it may be extended beyond the limits of the plotted points, it will usually be found sufficient.

Sources of Arithmetic and Logarithmic Probability Paper: Codex Book Co., Norwood, Massachusetts. Educational Exhibition Co., Providence, Rhode Island.

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RATIO CHARTS, for idrnlifirr Cro«« lmr« i|>iii-r rloirr As you l(M>k hichcr.

Chapter 41 RATIO CHARTS

^^ ratio chart is designed to indicate rate of change rather than arithmetic change. Although in many instances the spacing of the ruHngs clearly indicates to an experienced reader that the chart is plotted on ratio ruling, it is frequently desirable to indicate the ratio basis as shown in 345 and 346. This is especially necessary if the chart covers a comparatively short range of scale since the reader might not notice the difference in spacing of horizontal lines on the grid.

Synonyms for ratio chart are logarithmic chart, semi-logarithmic chart, rate-of-change chart.

The term "ratio chart" is short and expressive. There is need for a corresponding term equally expressive to designate charts planned on the usual arithmetic basis.