The Revised Index of the Volume of Mining
extent and direction of the month-to-month fluctuations were substantially accurate, the general level of the index was somewhat too low. The current revision was undertaken therefore in order to correct this discrepancy in level and, at the same time, to improve the several industry indexes. Chart i presents the revised mining index, monthly, for the period since August 1922, in comparison with the unrevised index which is shown for the interval since January 19I9. The tie-on between the old and the new index was made in August 1922, the closing month of the prolonged coal strike in the bituminous coal and anthracite fields. The two indexes were then practically coincident. The higher level of the revised index is accounted for in the main by revisions in the normal lines for output of anthracite and bituminous coal and for shipments of iron ore. The technique of the analysis has in general been similar to that of the revision of the index of the volume of manufacture which was discussed in detail in the May 1929 REVIEW. The revised index, like the old, is a weighted arithmetic average of relatives, the weights being based upon average values of the minerals over a period of years. The fuels and metals group indexes have been retained and are shown in graphic form on Chart 2. The fluctuations in the