MarketWatch reported companies in the S&P 500 lagged privately-held companies in sales growth between March and June 2012, according to a recent study by research firm Sageworks.

The MarketWatch reports says sales growth by public companies averaged 8.2% in the second quarter, down from 9.7% in the first quarter which lags the 10.1% sales growth in the second quarter for private companies, up from 7.9% in the previous quarter.

On the plus side for S&P 500 companies, they generated better returns in the second quarter, with an average net profit margin of 9.2%, up from 8.9% in the previous quarter. That beat out the 8% net profit margin for private companies, up from 6.6% in the previous quarter.
 
Analyzing results dating back to 2008, private companies have seen less compression and less volatility in their net profit margins.