Investors will no doubt be eagerly awaiting the third-quarter gross domestic product readings in the coming weeks. But top investor Kenneth Fisher has advice for them: Ignore it.

“GDP calculations are useful — to a degree. But they’re government-compiled, hence wonky, based on inherently flawed assumptions and surveys,” Fisher writes in an Interactive Investor column.  “GDP growth rates are imperfect, backward-looking snapshots of the past. Example: the first US second-quarter 2011 GDP growth estimate (released in July) was 1.3%. It was revised down to 1.0% in August and markets fell. In September, it was revised up to 1.3%, beating expectations of 1.2%, and markets cheered that day. But why? We just ended up back where we started in the first place!”

What’s more, Fisher notes that the government will often revise GDP data years after the initial readings are released — sometimes significantly. “GDP isn’t worthless, of course,” he says. “It allows rough comparisons among nations. And, there is value in the transparency of the underlying data. But if GDP can move 1% or 2% four years later, you shouldn’t fret little revisions now. And you shouldn’t take GDP calculations as hard truth. All the joy or anxiety is silly — you can’t be sure the data’s even right.”

Fisher says third-quarter GDP is likely to be better than expected — and better than initial estimates. “We just won’t know it for a few years,” he says. “[So] bet against the doomsayers. Plan your trades now. Buy stocks that should do well in an economy that’s growing faster than most fathom.” He offers a couple of his picks, including Royal Caribbean Cruises.