For the US it has 3 citations:
Glazer, Nathan. American Judaism (Second Edition); Chicago: University of Chicago Press (1972); pg. 114.
"In 1947, a national public opinion poll had shown that 18% of Jewish respondents attended service once a month (the comparative figures were 65% for Protestants and 85% for Catholics). " [Other sources show Catholic U.S. pop. for 1948 at 19.9%; 19.9% * 85% = 16.92%]
Greeley, Andrew M. The Catholic Myth: The Behavior and Beliefs of American Catholics. New York: Charles Scribner’s Sons (1990); pg. 266, 269.
Pg. 266: "using the ISV [International Study of Values, 1981] and ISSP [International Social Survey Project, 1985] surveys… "; Pg. 269: "…proportion of men & women over 35 who attend Mass at least once a month… Spain, 77%; the United States, 75%… " [USA: 75% * est. 25% of pop. which is Catholic, (or est. 55 million) = 18.75% of total pop. attending mass monthly]
Neusner, Jacob (ed). World Religions in America: An Introduction; Louisville, KY: Westminster/John Knox Press (1994); pg. 94.
"About a quarter of the American population is Catholic; of this number, half attend church regularly (at least several times a month) and another quarter attend at least once a month. " [50% + 25% = 75% "churchgoers. " 75% * 25% = 18.75% of U.S. pop.]
That last citations looks as though 18% of Americans attend a Catholic Mass and 75% of Catholics attend either once a month or several times a month. Statistics always seem like Lego to me, you can build what ever you want out of them