Almost 90% of the converts, as stated on Amy Welborns blog in the comment boxes, are convesions via matrimony, and I read that some Bishops have said they lose almost helf of these converts within a couple of years.
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.
I can only comment for the group I studied with. I am going in alone; my husband is remaining Protestant for now. In my class were 4 single women, one man who had been baptized but never confirmed, a young man joining on his own, one young woman who was going through the class with her finacee who is already Catholic, and one man whose wife is Catholic. This is only for a portion of my RCIA group, as there are 60 total and we meet at different times during the week.
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.