In subproject C of the SILENCE project there is a task to study possibilities to replace some coast-by measurements made outdoors with either CPX measurements (also made outdoors) or laboratory drum measurements. This report is a background document intended for the Milestone decision regarding this topic. The report starts with a brief description of each method, including examples of equipment, and continues with an analysis of the characteristics of each method. The correlations between the methods, as obtained in comparative experiments, are studied. It is, for example, found that the laboratory drum method is representative of road measurements only if the drum surface is equipped with a material giving a texture similar to that of the road material. Even better is if the drum can be equipped with segments of the road material itself, which is possible for a couple of drums in the world; although one interesting option is that replicas of the same road surface (for example one according to ISO 10844) can be made to cover a great number of drums all over the world with an almost identical surface. Then the coast-by and drum methods are compared in more detail. The uncertainties connected with measurements made with the two methods are discussed and quantified. When all uncertainties are combined into an overall uncertainty it is concluded that the drum method seems to be slightly better than the coast-by method. As a result of the positive evaluation of the drum method, it is concluded that some types of measurements which are today made with the coast-by method, or future measurements which would normally be made with the coast-by method, may better be made with the laboratory drum method. This has many practical implications, since indoor measurements can be made independent of weather conditions and climate. Consequently, it is recommended to develop and specify a laboratory drum measuring procedure and to use it in SILENCE for certain of the applications; although the coast-by method will still be used for a great number of basic measurements.