A new evaluation technique for the centroid drift velocity W_r of isolated electron swarms in gases is proposed based on moment equations. In addition to the average velocity W_v of the individual electrons in an electron swam, W_r has another velocity componemt due to effects of ionization and eleotron attachment. Mechanisms that cause the difference between W_r and W_v are illustrated, and quantitatively demonstrated by simulations using a propagator method. The effects of ionization and electron attachment on W_r are analyzed in detail by introducing the moment generation rate as a key quantity to categorize the effects.