Teaching About Religion: U.S. Demographics

Example Maps by Percents and Quartiles

State-by-State Percent Maps

Sometimes a straightforward portrayal of distribution by percent is
practical. In the case of a small religion, the percentage of population by
state can be low and have few values. Numbers might only range from zero
to five percent, for example. When that is the case, a map can simply
indicate those states having each of the percent values. States where
representation rounds to 1% are in red, 2% is shown in green, and so forth.
One can quickly spot the states having higher and lower concentrations of
that given religion.

(In the map above, states shown pale green have negligible
representation).

State-by-State Quartile Maps

To make comparisons within a large population, it is not practical to use a
different color for each different percent. One might need 25 or more
different colors, and the displayed results would be difficult to interpret. This
problem can be addressed by using quartiles.

The map (with table) below exhibits the demographics of a certain group in
quartiles. The representation is of a medium large group that has
membership in many states. The 48 states are colored to correspond to the
table. Consider North Dakota (colored blue). You can see that it has a high
population of group members within its general population, as do Arizona
and New Mexico, Illinois, North Carolina, Maryland, New York and New
Jersey, and some New England states. You can see that Oregon, Utah, and
numerous southern states (shown red) have a low percentage (1% to 14%)
of their population as group members.

This map shows four colors, one for each quartile of the group. But how
does such a map come about? Where did the table's numbers come
from? To illustrate, we will invent an imaginary religion that is a good
candidate for a quartile map. In fact, let's suppose the map above is for our
imaginary religion. We will call it the religion of Mystic Naturalism.

A Concrete (but Invented) Example

In the United States, the Mystic Naturalists number 24,009,224 total
adherents. Surveys have determined that this religion has representation in
all states (ranging from fewer than 2% adherents in some states to a high of
51% adherents in one state). We can rank the states, listing them in order by
these percents, from low to high (or the reverse).

One quarter of the Mystic Naturalists constitutes 6 million members (1/4 x 24
million). Beginning with the states having the lowest percentages of
adherents, we begin a roll and tally. We find that a number of the states must
be listed before six million of the MN adherents are included. As it turns out,
this 1/4 (quartile) constitutes members in states with a range of 1-14%
representation. We continue, and a second quarter (another 6 million) reside
in the states of the next larger representation grouping. (We had begun our
tally anew with states at 15% and end that quartile at 23%, with half the
adherents now included in the accumulation). A third quartile (yes, 6 million)
encompasses states with 24-28% representation, and the last 1/4 of the
adherents are in states having 29-51% of adherents. The states of the map
are then colored to represent these four quartiles.

Quartiles can look "strange" at times. Whereas our Mystic Naturalism
members are in all states, some other religion's membership might
distribute quite differently. For example, if members of the Spiritual Avatars
reside primarily in one state (for example Colorado, at 61%), then that state
alone would contain more than the largest two quartiles, with the remainder
distributing across other states of the nation. Quartiles would not be a good
choice of statistic for this situation.

When you return to the Demographics page, you may want to look at
Catholicism as a representation of a quartile map, and Judaism as a
percent map.