According to Cindy Mushet, author of the Art & Soul of Baking, many home ovens are off by 25-50* even when brand new. A difference of this amount can easily affect your baking outcomes so it is important to test your oven's temperature periodically.
To Test Oven Temp:
Purchase an alcohol based oven thermometer. Adjust your oven rack to the center and place the thermometer in the center of the rack. Set the over temp for 350* and allow time for it to fully heat up. Many ovens will beep when it has reached the set temperature, but don’t believe it. Allow for at least 20-30 min before you open the door quickly and read the temperature on the thermometer. If the temperature is hotter or cooler than the temp you set the oven to, make a note of it and you will be able to adjust for this later in yoru baking. For instance if you know that the oven is 25* cooler than what you set it for, you will know to set the oven to 375* when a recipe calls for 350*.
In order to keep your temperature accurate, be sure to check the gasket around the door to make sure that it's attached and properly fitted. A loose gasket will allow heat to escape. Also be sure that no vents are obstructed.
Good luck and have fun!
Marcie and Katelyn