Via Goldman:
All of these variables are related. Tight labour markets are typically associated with higher inflation expectations. These, in turn, tend to tighten policy and weaken expectations of future growth. High valuations, at the same time, leave equities vulnerable to de-rating if growth expectations deteriorate or the discount rate rises, or, worse still, both of these occur together.
To aggregate these variables in a signal indicator, we took each variable and calculated its percentile relative to its history since 1948. For the yield curve and unemployment we took the lowest percentiles relative to history, while for the other indicators we took the highest. We then took the average of these.