Critics of that dovish view charge that slack in the labor market is misleading since those outside the full-time labor force can't do the tasks most in demand—write code, frack wells and so forth. The same argument could be made for plants and equipment. While the headline numbers in Friday's report on industrial-production growth and utilization won't answer that, the data behind them will provide clues.

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Economists surveyed by The Wall Street Journal see utilization having stayed at 79.1% in July, unchanged from the prior month but tied for the highest since June 2008. That is a normal level based on long-term averages.

 Look below the surface, though: Electric utilities were at 78.7% in June, well below their long-term average of 87.4%. Idle or underused power plants don't depress utility bills.
Meanwhile, utilization for transportation equipment was 79%, well above the 74.4% long-term average. What's more, subdued capital investment in industrial machinery in the past six years may have left part of the excess as obsolete, creating potential supply constraints.

 Michael Shaoul, chief executive of Marketfield Asset Management, notes the 80% level of industrial-capacity utilization should be crossed in early 2015 at today's pace of growth. But he says that even the current rate historically has begun creating inflationary pressure. And he notes that economists may be underestimating how high the effective rate of utilization is given obsolescence.

 Looking at headline numbers alone, neither unemployment nor industrial-production data have done much to undermine the camp arguing that rates should remain "lower for longer." Bidding wars for pipe fitters and triple shifts at General Motors Co. plants, meanwhile, can be dismissed as anecdotes for now.

 If that changes and inflation begins to pick up, the cause shouldn't be a surprise. The economy has the wrong kind of slack.