ad lib vs do something off the top of one's head

ad lib

adj

Extemporaneous, impromptu.

verb

To perform without preparation.
To perform without script.
To perform extemporaneously.

adv

Extemporaneously.
At will.
To whatever extent.
At pleasure.

do something off the top of one's head

How often have the words ad lib and do something off the top of one's head occurred in a corpus of books? (source: Google Ngram Viewer )