Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE chip_model ( model_name text, launch_year number, ram_mib number, rom_mib number, slots text, wifi text, bluetooth text )
TABLE: CREATE TABLE screen_mode ( graphics_mode number, char_cells text, pixels text, hardware_colours number, used_kb number, map text, type text )
TABLE: CREATE TABLE phone ( company_name text, hardware_model_name text, accreditation_type text, accreditation_level text, date text, chip_model text, screen_mode text )

### Question ###
Find the average ram mib size of the chip models that are never used by any phone.

### Accurate SQL ###

SELECT AVG(ram_mib) FROM chip_model WHERE NOT model_name IN (SELECT chip_model FROM phone)