A rough order of magnitude (ROM) estimate is the least accurate estimate. The PMBOK Guide 4th Edition gives the guidlines that ROMs are -50% to +50% accurate, the PMBOK Guide 5th Edition gives the guidelines that ROMs are -25% to +75% accurate, or potentially even larger.
Rough Order of Magnitude (ROM) Estimate versus Definitive Estimate. Depending on the organizational structure in which you’re managing projects and the type of project you are working on, you may use different approaches to estimating. Two common estimating types are ROM estimates and definitive estimates. A rough order of magnitude (ROM) estimate is the least accurate estimate. The PMBOK Guide 4th Edition gives the guidlines that ROMs are -50% to +50% accurate, the PMBOK Guide 5th Edition gives the guidelines that ROMs are -25% to +75% accurate, or potentially even larger. Part of the Mathematics glossary: An order of magnitude is an exponential change of plus-or-minus 1 in the value of a quantity or unit. The term is generally used in conjunction with power-of-10 scientific notation. In base 10, the most common numeration scheme worldwide, an increase of one order of magnitude is the same as multiplying a quantity by 10.  order of magnitude. English Collins Dictionary-English Definition & Thesaurus &nbsp.  n. pl. orders of magnitude. 1. An estimate of size or magnitude expressed as a power of ten: Earth's mass is of the order of magnitude of 1022 tons; that of the sun is 1027 tons. 2. A range of values between a designated lower value and an upper value ten times as large: The masses of Earth and the sun differ by five orders of magnitude. n. pl. orders of magnitude. 1. An estimate of size or magnitude expressed as a power of ten: Earth's mass is of the order of magnitude of 1022 tons; that of the sun is 1027 tons. 2. A range of values between a designated lower value and an upper value ten times as large: The masses of Earth and the sun differ by five orders of magnitude. Definition of ORDER OF MAGNITUDE. : a range of magnitude extending from some value to ten times that value. ADVERTISEMENT.  More example sentences. 1  To measure changes that take place in about a femtosecond, they needed something they could characterize in attoseconds, an order of magnitude lower than existing systems.  There is always the possibility of some variance from the estimate, but definitive estimates are -5% to +10% accurate. A definitive estimate is based on detailed information from each work package within the WBS or estimates completed at the activity level. A rough order of magnitude (ROM) estimate is the least accurate estimate. The PMBOK Guide 4th Edition gives the guidlines that ROMs are -50% to +50% accurate, the PMBOK Guide 5th Edition gives the guidelines that ROMs are -25% to +75% accurate, or potentially even larger. Rough Order of Magnitude (ROM) Estimate. A rough order of magnitude (ROM) estimate is the least accurate estimate. The PMBOK Guide 4th Edition gives the guidlines that ROMs are -50% to +50% accurate, the PMBOK Guide 5th Edition gives the guidelines that ROMs are -25% to +75% accurate, or potentially even larger. Rough Order Of Magnitude Estimate. When a project is started and Requirements are not specified in great detail, the exact project Budget can not be calculated without mistakes (due to the well-known Cone of Uncertainty). 
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order of magnitude estimate definition