Julie, Pat Garrow explained this general rule of 8 years to me several years ago. He quantified it at a site in Washington, D.C. and I think repeated it on a site in Phoenix, Arizona. Paul Hampson also informed me of similar findings, though I dont know where he reported it. I suggest you contact Garrow for a specific reference. Ron May Legacy 106, Inc. P.O. Box 503394 San Diego, CA 92150-3394 (619) 229-0648 [log in to unmask]