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The level starts at 100. 110 means the parts in this index cost about 10% more than at the start; 95 means about 5% less.

It follows price changes, not price levels: each month it moves by the typical change of the parts that were priced both that month and the month before, so a few very expensive parts never dominate it, and parts coming and going don't move it on their own.

1, 3 and 12 months show the change over that period. The barometer shows where today's level sits between the index's lowest and highest point.