Academic Ability Or Socio Economic Status 525270

Please help me explain the Miller’s experiment

The typical pattern should show that subjects have little difficulty with the 5- and 7-item strings, but STM weakens as the string lengths creep into the 9-12 item range.

What length of string did you have difficulty with? Do you think this would change with age of subject, academic ability or socio-economic status of subject, a difference in the way you read the string of digits, or something else?