what proportion of investors in the stock market are winners?

If by winner, you mean beating the market then 50% does better than the average and 50% does worse. This is by the very definition of the average of the market. Hence your best bet are always the no load, low fee index funds as they are the highest return for the effort and hence management costs.

I read the most thoughtful answer to this in an article by Robert Precther here: http://www.elliottwave.com/. He indicated, showing statistics, that over time the net gain for all investors amounted to about 3% of which is equal to Wall Streets commissions. I have no idea if this is true or if I misconstrued what I read.

I would like to know the exact raw answer to your very broad question too. I have known both winners and losers but the samples are too small and periods too diverse to be statistically correct. It is a difficult statistic to find because the industry, in my opinion, does not want you to know. The IRS or SEC might have a statistic on stocks only winners and losers for a specified period but I have not found it.

I am sure if you asked Schwab or Ameritrade, etc. to give you their customers credit balances for any specified accounting period including cash in, cash out and net buys and sells they would not do it but if you were an executive with one of those companies you might be shocked by what you see.

Additionally, if you research the subject further, you will often see the statement that most TRADERS loose money but that does not answer your question. It only says what a certain class of investors does. You may have to restate your question indicating to which period, or to which specific class of investor you are referring because some Stock Market investors make money all the time.