manufacturing quality, care, and the rigorousness of use. A poor quality foam comfort layer can deteriorate noticeably in 1 year, while a quality latex core can last 20 years or more; innerspring cores typically last around 10 years. The comfort layer is almost invariably the first area to fail, which is why mattresses are often double-sided, to extend the lifespan. A separate topper may be used instead of or in addition to a comfort layer, which reduces wear and is replaceable without replacing the entire mattress. The majority of high-end mattresses have a lifespan of between 7–10 years but it can last beyond 10 years and more depending on the level of care. In the United States, mattress warranties are typically for 10 years or 20 years, sometimes 25 years, though this specifically addresses manufacturing defects and faster-than-normal deterioration, not expected deterioration with time. In the United States, there is a general expectation that mattresses should last about 10 years, and this is the average number of years Americans keep mattresses, though this varies by age group. This expectation is based on a number of factors, including sales pitches; the expectation that mattresses will last the length of their warranty, hence 10 years or 20 years, accordingly; and comparison with other household items. The mattress replacement cycle is a key driver of income and profits for the mattress industry – a five-year replacement cycle yields double the sales of a 10-year replacement cycle, for instance – so the mattress industry has a financial incentive to shorten the replacement cycle. Notably, the International Sleep Products Association (ISPA) established the Better Sleep Council (BSC) in 1979 with the stated goal to "shorten the mattress replacement cycle", in addition to encouraging people to "invest in better bedding". An industry-funded 2006 study by researchers at Oklahoma State University (funded by the BSC) of 59 people with poor sleep who received free new replacement mattresses for their existing mattresses 5 years or older (average age 9.5 years) found improved sleep, particularly when the existing mattresses were cheap. A follow-up paper by some of the same authors with additional statistical analysis reinforced these conclusions. The BSC has subsequently cited this study in the ISPA-published news magazine for mattress manufacturers, BedTimes, to advocate a more frequent replacement cycle, specifically to "consider replacing a mattress every five to seven years"; the recommendation is based largely on this study.