Advances led declines

I love National Public Radio. Every evening on my way back from work I tune into on my jogging radio. I was strange to realize how I wait to hear the stock market news which concludes with the words “advances led declines by a ratio of X to Y.”

When I first heard those words I was surprised. What does it really mean?

Early on I ventured to speculate…
…May be some of the stocks that advanced lead the decline of others? May be market moved money from one set of stocks that at some point performed better to others that have stronger prospects today in the ratio of their numbers? But the hypothesis did not verify.
…Then I wondered may be early in the day stocks advanced and declined by close of market in the ratio of duration the day? Once again the hypothesis did not verify.
…Or may be a certain percent (X/X+Y) declined while the rest (Y/X+Y) declined?

I wondered what is worse? To be missing data or lack the know how to use it? All so common a story in corporations today.

(Advance/decline numbers give you an indication of market performance. The advance to decline ratio adds a dimension to the information conveyed by the index to spot trends.)