"Worst performance in 15 years" was the judgment of a half dozen researchers at the Federal Reserve when it came to financial conditions that could reduce economic growth.

The researchers were working on a new financial conditions index (FCI), a category of metric that is supposed to reflect what can be known from a collection of financial variables. The intent is to create a single number that can succinctly reflect a broad view of the economy, like the Federal Reserve Bank of Chicago's National Financial Conditions Index or the Bloomberg FCI.

"The main drawback of such an approach is that the weights used to aggregate the underlying financial variables are based on statistical models and do not necessarily have a straightforward economic interpretation, nor do they map changes in financial variables directly into measures of economic activity," they wrote.