Given a stock traded on a public exchange, how can one determine how much of the stock is owned by individual investors (vs. institutional investors)?

These methods could work globally:

• By looking at the annual reports of the company in question. There will typically be a section that lists the largest shareholders.

• By extracting the data from publicly-available company filings databases (e.g. EDGAR in the US). Such databases only exist in some countries. You may need to process the data using a computer to actually get the information you need.

• By looking at the holdings of many institutions (e.g. ETFs, mutual funds, etc.). You could extract information about the number of shares they collectively own. However, this requires a large database of institutional holdings.

• Get the information from a data vendor.

I think you should wait for a better answer, because I am not knowledgeable enough to give you more information.

From Investopedia:

• Insiders are a company's officers, directors, relatives, or anyone else with access to key company information before it's made available to the public.

• Form DEF 14A the proxy statement which lists directors and officers, and the number of shares they each own.

• Companies file Schedules 13D and 13G to disclose outside beneficial ownership information of more than 5% of a company's stock issue.

• Stock owners file Forms 3, 4, and 5 to disclose insider beneficial ownership when they have more than 10% of voting power.

Note that these are not pinpoint accuracy numbers. For example, anyone acquiring more than 5% of a company's stock has 10 days to file Form 13D with the SEC.

Ownership of less than 5% does not require a SEC filing. Figuring out institutional ownership below 5% is problematic. Some analysts use varying techniques to track the limitations of quarterly institutional disclosures. For example, "Investors Business Daily" provides daily and weekly volume information for all listed stocks and has a rating system for purchases and sales. Heavy accumulation implies institutional accumulation.

The NASDAQ has an Institutional Holdings which provides a summary and more detailed view of the aggregated institutional stock holdings, including owner names and ownership analysis.