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One things that has always troubled me when buying a sleeping bag is the temperature rating. When looking at a bag that claims to be a +15 bag (for example) how do I know that it is really good down to 15 degrees (preferably without buying it)?

Is the rating just determined by the manufacturer, or is there any sort of independent validation/criteria on these ratings?

One things that has always troubled me when buying a sleeping bag is the temperature rating. When looking at a bag that claims to be a +15°F bag (for example) how do I know that it is really good down to 15 degrees Fahrenheit (preferably without buying it)?

Is the rating just determined by the manufacturer, or is there any sort of independent validation/criteria on these ratings?