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National Aitociation of Motor Bus Operators. Washington, D. C, "Bus Facts for 1938." SCALE .9

B. Comparison of the Number of Cities Using All-Rail Transportation, Combined Rail and Bus Transportation, and All Bus Transportation in the United States from 1912 to 1937.

1. In this chart, as the number of cities using rail and bus transportation increased, the

number using only rail transportation necessarily decreased, since the number of cities included in the study varies little over a period of years.

2. In more recent years, as those using buses only increased, the number of cities using

the combination system decreased.

3. This type of record would be affected by any change in the total number of cities within

the three classifications. Similar charts might be based upon 100%. See 298A.

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GRAPHIC PRESENTATION

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Standard Statistics Co., 'Standard Trade and Securities." March 4. 1QJ8

A. The Price of Tin Per Pound in the United States from 1 93 I to 1 938.

1. The inverse relationship chart is an especially interesting one. If there is a causal

relationship, a rise in one curve may cause a fall in the other.