Keyword search phrases with four words are the most effective at converting browsers into customers. Four-word phrases for keywords in the top 100 group of words that produced the highest traffic rates converted 38.28 percent of unique visitors into customers in December 2004. Three-word phrases converted 21.89 percent of unique visitors into customers. The conversion rate for five-word phrases takes a considerable dip to 9.69 percent.

[Source:How Keyword Length Affects Conversion Rates]