US law states that every applicant for a non-immigrant visitor visa will be presumed to be an intending immigrant (overstay) to the United States, or as previous poster said a "scheme".
In order to overcome this presumption and qualify for a visa to the US, visitors must demonstrate
significant economic, familial, social and other ties to their
home country that will compel them to leave the US before the end of their authorized visit/stay.
Refusal rates differ- depending on applicant's nationality (as well as application location I guess). Some countries' citizens are notorious for remaining (overstaying), and with the current world economic downturn it has also had a negative impact on the ability of visitors to qualify for visas.
For example Taiwanese have one of the world's lowest refusal rates (3.2%)- regardless of where application is made. While on the flipside, Laotians have a 74.4% rate. These rates are for FY 2010. The rates are rumored to be significantly higher than advertised...