Dominican is not an adjective (at least not a regular one) for the People from the Dominican Republic. It is a nick name given in 1621 by the Spanish Crown to the Spaniards descendants born in the Island of Santo Domingo. That nick name was used for Dominican-Spaniards, who owed allegiance to the Spanish Kings and Queens. Once the Dominican Republic was established in 1844, this nick name continued to be used as attached to the name of the Island of Santo Domingo, but not as a national identity, as per the first (1844) Constitution. Later in the years, the Constitution was modified silently and obviously ignorantly, by using Dominican Republic as the name of the country.
Juan Pablo Duarte and the Trinitarians were clear about the name of the country and the denomination of the State: two very different concepts. The name chosen by Duarte for the country was Quisqueya.
The denomination for the republic, based on the name of the Island, to oppose the name of the Island shown in the Haitian Constitution, was the Island of Santo Domingo.

For the United States of America, "Dominican" is a national from the Island of Dominica. Showing this in the Census form would have created confusion in the results. That's why the name "Dominican" appears under the classification of "Other" and will be grouped with other ethnic groups.

If DANR (The Dominican National Roundtable) had considered our proposal, we would have at least included "Quisqueyan" in the group list, waiting for confirmation in our Constitution that our real, true and proper National Identity is that of Quisqueyan.

A common nick name shouldn't be our national identification. We can claim this less and less to differentiate from Haitians, meaning that we are from the Island of Santo Domingo with a Hispanic cultural background, but it is easier to call ourselves Quisqueyan, meaning that we are from a country named Quisqueya, occupying the two third Eastern part of the Island of Santo Domingo.

We have stressed before that the name of the Island is not Hispaniola or Haiti. See http://www.quisqueya.name (Spanish) for more information.

Interesting point