"Fixed" population values in ACS 5-year estimates

I have recently noticed that some ACS 5-year tabulations contain missing margins of error for some estimates. For example, this occurs frequently in the dataset for population by nativity for the Hispanic population (ACS table B05003I). I have looked online and found documentation explaining what those missing margins of error indicate and how to treat them:

If the margin of error is displayed as ‘*****’ (five asterisks), the estimate has been controlled to be equal to a fixed value and so it has no sampling error. A standard error of zero should be used for these controlled estimates when completing calculations, such as those in the following section.

However, I haven’t had any luck finding further explanation of where the need for these fixed values comes from, nor why this seems to happen only in the Hispanic population (based on several datasets I’ve looked at). Does anyone have insight into this?

These fixed values refer to official population estimates generated by the Census Bureau from the Population Estimates Program (PEP); these are also the controlled estimates referred to in this Census training presentation. The PEP produces official population estimates using administrative sources such as IRS, Social Security, and Medicare/Medicaid records. Since the ACS samples only about 1% of the US resident population in each year, counts from the survey need to be extrapolated to produce tabulations that are representative of the entire population. The PEP values are used to create weights in the ACS so that the weighted estimates of housing units and persons by age, sex, race, and Hispanic origin conform to estimates from the PEP of the Census Bureau at a specific point in time (see Population Controls in the ACS User Handbook). Since these are official estimates, there is no associated margin of error.

This pattern should only occur for statistics that correspond directly to PEP values. If you do find exceptions, you could consider posing the question on the ACS Data Users Group forum or sending it directly to the Census Bureau.

Thanks Ivan, this is really helpful!