I did a test and would appreciate feedback from experienced people to help me understand how the geocoding and spatial merge processes work.
Test objective: compare census tracts obtained when using only zip information (9 digit) to those obtained with the full address (street name and number, city, state, zip, zip4). The test used the same set of input records; one time ArcMap received the full set of inputs, the other time it only got zip and zip4. Both times the results of the geocoding step were used as input to a spatial merge to get the census tracts. In both cases we are using the ZIP4_LocalComposite address locator from StreetMap Premium.
There is a 91% agreement on the results (comparing the combined state, county, tract, block group codes). The part I don't understand is why there are cases where the state code is different? They are very few (9 out of over 145 thousand records), sometimes the correct code comes from the results using the full address, but other times the correct state codes come from the records geocoded using only the 9 digit zip codes.