Attribute plans are generally easier to use than variables plans. A sample of n units is selected randomly from a lot of N units or from ongoing production. If there are c or fewer defectives, accept the lot. If there are more than c defectives, reject the lot. For example, suppose you have a shipment of 10,000 bolts. You will inspect 89 of them. If there are 0, 1, or 2 defective bolts, then you may accept the shipment. If there are more than 2 defectives, then reject the entire lot of bolts.
For variables sampling plans, you can only examine one measurement per sampling plan. For example, if you need to inspect for wafer thickness and wafer width, you need two separate sampling plans. Variables sampling plans assume that the distribution of the quality characteristic is normal. However, the main benefit from using variables data is that a variables sampling plan requires a much smaller sample size than an attributes sampling plan.