Nineteen Sixty-four is a research blog for the Center for Applied Research in the Apostolate (CARA) at Georgetown University edited by Mark M. Gray. CARA is a non-profit research center that conducts social scientific studies about the Catholic Church. Founded in 1964, CARA has three major dimensions to its mission: to increase the Catholic Church's self understanding; to serve the applied research needs of Church decision-makers; and to advance scholarly research on religion, particularly Catholicism. Follow CARA on Twitter at: caracatholic.

10.09.2009

Measuring Up: Aggregating data to estimate the number of Catholics at the state level

How many Catholics live in your state?

A recent CARA analysis of data from multiple sources indicates that in most states there may be more Catholics than the Catholic Church is aware of. 

Each year in The Official Catholic Directory (OCD), dioceses report a variety of statistics—including their estimates of the total Catholic population.  The 2009 OCD is intended to be representative of the “status of the Catholic Church as of January 1, 2009.” At CARA, we interpret these data to be reflective of totals in 2008, when these data are collected.

Recently some very large surveys regarding religious identification have been conducted which allow for population estimates at the state level. The first of these was conducted in 2007 by the Pew Forum on Religion & Public Life. The U.S. Religious Landscape Survey included interviews with a representative sample of 35,556 adults in the United States. The second source used here is American Religious Identification Survey (ARIS) conducted by researchers at Trinity College in 2008.  This survey included interviews with a representative sample of 54,461 adults in the United States.  Surveys with such large sample sizes allow for state level estimates of religious identification that are not possible with a typical academic or media poll that may only interview 1,000 respondents nationally.

Both of these large surveys estimate the proportion of the adult population (age 18 and older) that self-identifies their religion as Catholic.  These surveys likely underestimate the Catholic percentage of the total population—including children—as Catholics in the United States are, on average, younger than the non-Catholics (and more likely to be of childbearing age) and Catholics have a higher fertility rate than non-Catholics (both of these demographic factors are related to immigration).  Thus, the percentage of the population under age 18 in the United States that is Catholic is likely greater than the percentage of the U.S. adult population that is Catholic. 

In addition to these surveys, estimates are available from the Association of Statisticians of American Religious Bodies (ASARB).  This organization produces county-level estimates for religious adherents of all ages that coincide with each U.S. Census. Adherents represent any members of the faith, regardless of attendance at services, confirmation, etc.  The most recent estimates from ASARB represent the year 2000. Due to the timing of this data collection, the ASARB estimates, in some cases, misrepresent Catholic population percentages due to changes that have occurred in the last nine years—primarily any significant Catholic mobility (moving from one state to another) and any significant Catholic immigration (Catholics coming to the United States from other countries).

Despite the methodological and timing differences between these four estimates of the Catholic population percentage (i.e. including the OCD), each is strongly correlated to all others at the state level (Pearson’s R >= .915).  

CARA has aggregated these Catholic population estimates by simply averaging them (Note: not all states have estimates from all four sources). This is represented as "Average" in the tables below. We use this average to estimate the total size of the Catholic population in each state based on U.S. Census Bureau estimates of state population size in 2008. This is represented by "Catholic Population Estimate" in the tables below (i.e., multiplying the "Average" by the "Total Population" and creating the estimates in the "Catholic Population Estimate" column. Note: the "Average" includes additional decimal places not shown in the table. Thus, multiplying the table figures for "Total Population" by "Average" will produce slightly different estimates of "Catholic Population Estimate" than what is shown in the table. The complete data are available upon request). 

Using this method, the total estimated size of the self-identified Catholic population (i.e., adults and children) in the United States in 2008 was approximately 70.5 million (by comparison the OCD estimated this to be 65.2 million for the 50 states and Washington D.C.).  This represents 23.3% of the total U.S. population of 303.2 million in that year.

The table below includes states where the Catholic population percentage estimated from the OCD is relatively consistent with estimates made from the surveys and ASARB. 

The next table includes states where the Catholic population percentage estimated from the OCD is either significantly underestimated or overestimated in comparison to the estimates made from the surveys and ASARB.  This table also includes those states where no OCD estimate is available due to diocesan boundaries (e.g., Arizona and New Mexico, Delaware and Maryland).

The outlier states where dioceses have likely underestimated the size of their Catholic population are: Mississippi, Florida, Maine, North Carolina, Tennessee.  In each of these states the size of the Catholic population is estimated to be more than 50% larger than what is reported in the OCD based on the most recent survey estimates. Florida in particular is estimated to have more than 1.4 million more Catholics than what is reported in the OCD (2,255,891 reported in the OCD compared to CARA's estimate of 3,729,817).

The only dramatic outlier state where dioceses have likely overestimated the size of their Catholic population is Nevada where the estimated size of the Catholic population is 25% smaller than what is reported in the OCD based on the most recent survey estimates.

Even the aggregated estimates shown above, for a variety of reasons, likely underestimate the Catholic population in general. As noted, ASARB is based on data collected in 2000 and the figures reported by dioceses in the OCD are often conservative and most consistent with ASARB estimates. The surveys by Pew and ARIS include only adults. Finally, one of the most difficult populations to survey and count in the United States are immigrants. Many recent immigrants to the United States have come from countries in Latin America and Asia that have large Catholic populations. Immigrants in general are known to be less likely to agree to be interviewed in surveys and may also be under-sampled due to the lack of a landline phone (i.e., mobile phone-only households). Researchers using Census Bureau surveys often make adjustments for this "undercount" (see Pew's, A Portrait of Unauthorized Immigrants in the United States). To the degree that Catholic immigrants are missed in surveys, the Catholic population percentages estimated using these data are by definition underestimated.

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