Active share is a relatively new calculation1 and provides a measurement of whether or not a portfolio “hugs” an index or not, which is an indication of how actively a portfolio is being managed. An active share of 0% is a portfolio that precisely matches the index. Therefore a portfolio with a low active share contains many of the stocks held in the index and can be considered to be passively managed by a “closet-indexer”. A portfolio that has an active share of more than 60% can be considered to be actively managed as its holdings mirror the index to a far lesser extent – a 60% (or more) difference. This indicates that the portfolio manager is managing the fund independently of the index, finding his or her own investment ideas to put into the fund.
See Pender’s Active Share scores
 Cremers, Martijn, and Antti Petajisto. 2009. “How Active Is Your Fund Manager? A New Measure That Predicts Performance.” Review of Financial Studies, vol. 22, no. 9 (September):3329–3365.