Answer:

Till Jenisch of Pforzheim, Germany sent in this question a few weeks ago. Quoting directly from Mastering Elliott Wave, page 2-9, "The Wave Theory requires a large degree of public involvement...to manifest itself. Since Cash markets always involve more participation by the public (through direct buying and consumption, as is the case with commodities) than Futures markets, wave formations always tend to be more standardized and predictable when cash data is used. In constructing your data series, use cash data whenever possible."

What the above means is wave development and analysis is easier and more accurate on a cash chart (except in some agricultural markets where no universally accepted cash price is available). Unfortunately, in most cases (excluding Forex markets), the public cannot easily trade cash, leaving Futures markets their only choice. So, if you want to take advantage of a trend based on a wave forecast, you will get the best results by doing your analysis in the cash market (using wave charts, not bar charts - see MEW page 2-12) then placing your trade in the Futures market, making the translation between cash and futures the day the trade is entered.