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or scaling ratio to generate national estimates from NFIRS data But NFIRS is a sample from a universe whose size cannot be inferred from NFIRS alone. Also,participation rates in NFIRS are not necessarily uniform across regions and sizes of community, both of which are factors correlated with frequency and severity of fires. This means NFIRS may be susceptible to systematic biases. No one at present can quantify the size of these deviations from the ideal, representative sample,so no one can say with confidence that they are or are not serious problems. But there is enough reason for concern so that a second data base-the NFPA survey- is needed to project NFIRS to national estimates and to project different parts of NFIRS separately. This multiple calibration approach makes use of the annual NFPA survey where its statistical design advantages are strongest There are separate projection formulas for four major property classes(residential structures,non-residential structures,vehicles, and other)and for each measure of fire severity (fire incidents,civilian deaths,and civilian injuries, and direct property damage). For example,the scaling ratio for 1998 civilian deaths in residential structures is equal to the total number of 1998 civilian deaths in residential structure fires reported to fire departments, according to the NFPA survey (3,250), divided by the total number of 1998 civilian deaths in residential structure fires reported to NFIRS(1,224). Therefore,the scaling ratio is 3,250/1,224— 2.66. The scaling ratios for civilian deaths and injuries and direct property damage are often significantly different from those for fire incidents. Except for fire service injuries, average severity per fire is generally higher for NFIRS than for the NFPA survey. Use of different scaling ratios for each measure of severity is equivalent to assuming that these differences are due either to NFIRS under-reporting of small fires,resulting in a higher-than-actual loss-per-fire ratio,or possible biases in the NFIRS sample representation by region or size of community,resulting in severity-per-fire ratios characteristic only of the oversampled regions or community sizes. Note that this approach also means that the NFPA survey results for detailed property-use classes(e.g.,fires in storage structures)may not match the national estimates of the same value. Calculating National Estimates of Particular Types of Fires Most analyses of interest involve the calculation of the estimated number of fires not only within a particular occupancy but also of a particular type. The types that are mostly frequently of interest are those defined by some ignition-cause characteristic. The six cause-related characteristics most commonly used to describe fires are: form of the heat that caused the ignition, equipment involved in ignition, form or type of material first ignited,the ignition factor that brought heat source and ignited material together,and area of origin. Other characteristics of interest are victim characteristics, such as ages of persons killed or injured in fire. For any characteristic of interest in NFIRS,some reported fires have that characteristic unknown or not reported. If the unknowns are not taken into account,then the propensity to report or not report a characteristic may influence the results far more than the actual patterns on that characteristic. For example,suppose the number of fires remained the same for several consecutive years, but the percentage of fires with cause unreported steoodily declined over those years. If the unknown-cause fires were ignored, it would appear as if fires due to every specific cause increased over time while total fires remained unchanged. This, of course,does not make sense.