The Record High Percent Index is a market breadth indicator created by dividing the number of 52-week highs for a given market by the sum of the number of new highs and the number of new lows.

Record High Percent = New Highs / (New Highs + New Lows)

The values range between 0.0 and 1.0. A value of 0.0 means that there were no new highs on that day. A value of 1.0 means that there were no new lows on that day. A value of 0.5 means that the number of new highs and new lows were equal.

StockCharts.com computes and publishes three versions of the Record High Percent Index - one for the NYSE ($RHNYA), one for the Nasdaq ($RHCOMPQ), and one for the American Stock Exchange ($RHXAX).

More information on the Record High Percent Index is available in this ChartWatchers article.