Undercooking frozen or refrigerated convenience foods could make you sick. And if you use a microwave to prepare packaged foods, as 71 percent of Americans do, according to a recent Food & Health Survey , it’s crucial that you get one that heats food evenly. Consumer Reports latest tests of microwaves found fewer models that aced our evenness test.

An estimated 76 million cases of foodborne illness occur annually in the U.S., making them more common than you’d think, according to the Centers for Disease Control and Prevention. The USDA’s “Cook it Safe” campaign aims to reduce foodborne illness caused by consumers undercooking frozen and refrigerated convenience foods.

When food isn’t cooked evenly to an internal temperature that kills harmful bacteria that might be present, illness can result, according to the USDA. So using a microwave that delivers even heating is important. If you’re shopping for a countertop or over-the-range microwave, choose one from our microwave oven Ratings that scored Excellent or Very Good in heating evenness. Those with lesser scores left cool spots in cooked foods, which the needed extra cooking time.

We all find smart ways to save time in the kitchen and convenience foods are definitely handy, but here’s what the USDA says are some common mistakes when preparing them:

Not following cooking directions. Obvious, but 39 percent of people don’t follow all cooking directions, according to the Food & Health Survey. If the directions include allowing the food to stand a few minutes after removing it from the microwave, do so. This allows the food to continue to cook. Stirring the food midpoint, even if the microwave has a turntable, also contributes to even cooking, and so does covering food, which traps moisture and increases the temperature.
Ignoring the wattage. You’ll need to cook food longer if your microwave’s wattage is lower than the cooking instructions requires. Our Ratings indicate wattage, and you’ll find it on the serial number plate on the back of the microwave, inside the microwave door, or in the owner’s manual.

The USDA also recommends using a food thermometer to test food in several spots, but the survey found most people don’t, and nearly a third said nothing would change their mind. Using a food thermometer is a good idea, but at the very least, make sure there are no cold spots in your food.