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Chem 108, Long Measured and Exact Numbers Measured Numbers All of these numbers have uncertainty in the last digit on the right (ie. the last digit on the right was estimated. Significant figure rules must be used in writing these numbers and in rounding off answers to problems in which these numbers were used. Types: 1. Numbers measured with a measuring device 2. Numbers estimated by some other means 3. Numbers in a conversion factor showing a relationship between two different measurement systems (SI and English). Except that 2.54 cm = 1 inch was redefined as exact Exac t Numbers There is no uncertainty in these numbers. Significant figure rules are not applied to these numbers, either when writing them or when using them in problems. Types: 1. Counted numbers (where the numbers are small enough to accurately count) 2. Numbers in a conversion factor showing the relationships between units within the same measurement system (defined conversions) Note: You do not apply significant figure rules to a 1 on one side of a conversion factor. The 1 in a conversion factor used in a problem is considered to be as accurate (have as many significant figures) as the number to which it is related on the other side of the factor.