One type of test is the retention test. Retention is the capability of a cell to maintain the programmed information over time. The specification requests ten years of retention at beginning of life and one year at end of life. The evaluation of the retention gives a very good idea about the maturity and capability of the flash technology. The diagram shows the distribution of bit errors for each individual flash page. The dotted line shows the distribution at the beginning of life, the dashed line after an equivalent of 10 years, and the solid line at an equivalent of 43 years of storage. First of all, the different distribution lines hardly degrade over time, which is a good indicator for the cell stability. Even after an equivalent of 43 years of storage, the bit failure rate of the worst pages only requires about 25% of the error correction capability, leaving a lot of margin before the first uncorrectable errors will occur. These margins can be used for future program/erase cycles.