http://www.conversiondoctor.com/conversion-blog/coupon-codes...

  TEST: (Straight A/B test for an online retailer in the women’s clothing market)

  Control: Coupon code on the first page of the checkout process.

  Variation 1: Coupon code removed.

  Results:

  Control: 3.8% conversion rate. (967 sales / 25,489 unique visitors)

  Variation 1: 5.1% conversion rate. (1,276 sales / 24,991 unique visitors)