Survey data can be skewed in competition based incentive surveys

Some thoughts...
A popular way to get customer feedback about their shopping/product/service experience is to have them fill out a survey. These surveys typically have incentives such as a free product/service, discount coupon or some other prize. More recently, surveys are administered online, as opposed to having customers fill them out on paper and mailing them in. Soliciting customer feedback in this way is a very cost effective approach. However it has a few drawbacks:

1. A particular type of customer is more likely to fill out these surveys. Typically a young student or lower income class.
2. Based on how the survey is to be filled out (internet vs. mail-in) will also determine the type of customer that responds.

this will result in the survey data being skewed to over represent this particular type(s) of customer. One just needs to be aware of this effect and have other avenues to collect customer feedback for the other customer types.