Approximate Quantities
In Estimation, approximate quantity method
is regarded as the most reliable and accurate for estimating, provided there is
sufficient information available. It involves grouping items corresponding to a
sequence of operations and relating them to a common unit of measurement. This
approach is particularly useful when the surveyor has enough experience and can
quickly carry out measurements using composite rates to save time. While the
rules of measurement are simple, it's important to note that they are not
standardized and may vary slightly from one surveyor to another.
In this method, composite rates are built using data available in the office for a specific sequence of operations. All measurements are taken as gross, except for very large openings. Initially, it may take some time to build up the composite rates, but once they are calculated, they can be used for various estimating needs. This approach proves to be efficient and time-saving, especially when dealing with repetitive elements in construction projects.
In the past, this measurement method was typically carried out manually. However, with the advancement in technology, there are now reasonably priced software packages available that can assist in performing these calculations, further streamlining the estimation process.
As an example, let's consider the substructure composite rate for excavating a trench with a width exceeding 0.30 m and a maximum depth not exceeding 1.00 m. The composite rate includes various elements such as earthwork support, filling to excavations, disposal of excavated material off-site, in situ concrete foundation poured against earth, 275 mm cavity brickwork to 150 mm above ground level, bitumen-based damp-proof course, and external facings. The cost is given per linear meter of trench.
Normally, if measured in accordance with SMM7 (Standard Method of Measurement for Building Works), each of these items would be measured separately as described in Chapter 3. However, in the case of approximate quantities and when using this composite rate method, all these elements are combined into a single rate, which is then applied to each linear meter of trench.
It is worth noting that while this method
offers efficiency and speed, it is essential to have sufficient data and
accurate information to ensure the reliability of the estimates. Additionally,
as mentioned earlier, slight variations in the measurement approach may exist
among different surveyors due to the lack of standardization in this method.
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