I don't know how this could possibly be shown, since no statistics are kept on it.
As for people who leave the church, most never contact their parish to say they are no longer going, so many Catholics are still on the parish/diocean rolls but havent stepped foot in a church in years.
Not true. Parishes take annual censuses and purge their rolls of those who have moved or who no longer attend. That's part of what those little offering envelopes are for. You really think they just make up all those numbers of parishoners reported every year without ever considering people who might have signed up and haven't been seen in 10 years? With the way Americans move about, it wouldn't be many years with that sort of system before the numbers were utterly exaggerated, and made no sense at all compared to the number of people recorded receiving the sacraments.
Here is an example, and it was from a few years ago. The San Jose diocese technically has 600K on its lists, and at best, weekly mass attendence is around 100K, like most diocese, it does not really purge parishioners from its lists. In Boston, the nembers who attend mass on a weekly basis make an even smaller percentage.
I know its a disturbing concept to many, but the Catholic churhces numbers overall are inflated by quite a bit.