Hello, I need help locating governmental, industry and/or academic sources that would show the average employment length for stockbrokers in US and of stockbrokers at Merrill Lynch US specifically.

Hello,

I need help locating governmental, industry and/or academic sources that would show the average employment length for stockbrokers in US and of stockbrokers at Morgan Stanley US specifically.  

I have located this source https://www.zippia.com/morgan-stanley-careers-7713/, https://www.zippia.com/security-industry-specialists-careers-37922/ indicating a 4.3 avg length, but I need official sources (governmental, industry related e.g. FINRA, SIMFA, SEC, or academic research papers or studies) that would back up this result or indicate a different value that I could quote.

Thanks,

Violet