Why is it the case that in the 1970 Census, “metropolitan and central city status is not available for the entire states of Arizona, Delaware, Hawaii, Idaho, Maine, Montana, Nevada, New Hampshire, North Dakota, Rhode Island, South Dakota, and Utah; plus the rural parts of Arkansas, Colorado, Iowa, Kansas, Kentucky, Louisiana, Minnesota, Mississippi, Nebraska, New Mexico, Oklahoma, Oregon, West Virginia; plus the urban parts of Connecticut and Maryland.”
Was this done for confidentiality reasons? Were these data just not collected? Would you recommend using rural/urban status as a proxy?
The 1970 Census confidentiality rules required that households in states with rural and/or urban populations less than 250,000 be classified as N/A. There is more information about this on page 4 of the Description and Technical Documentation from the Census Bureau. It is on page 22 of the same document that you can see which states were determined to have less than 250,000 people in the rural classification, the metro classification, and both.
Unfortunately, you will find the 250,000 rule was enforced for the urban/rural status variables as well. It is up to the researcher to decide how to treat these missing states.