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Evaluation of the accuracy and consistency of the estimates led to the following conclusions:
Data from NWS files and other sources made it possible to reconstruct state and national flood damage estimates for 1976-79. However, little data was collected during 1980-82 and large errors were discovered in estimates developed later for that period. As a result, the years 1980-82 have been excluded from the reanalyzed data sets. Annual compilation of damage estimates resumed in 1983, but depended mainly on information from Storm Data in the first few years. Particularly in 1983-84, omissions are more likely and estimates probably contain larger errors because of the use of damage categories.
When damage in a state is estimated to be less than $50 million (1995$), estimates from NWS and other sources frequently disagree by more than a factor of two.
Errors tend to average out, as long as the local estimates are not systematically biased. When damage in a state is estimated to be greater than $500 million (1995$), disagreement between estimates from NWS and other sources are relatively small (40% or less). The relatively close agreement between NWS and state estimates in years with major damage is reassuring, since the most costly floods are of greatest concern and make up a large proportion of total flood damage.
Missing NWS estimates were discovered for floods in which the state claimed as much as $50 million damage (1995$). Such omissions would have little effect on national total damage estimates, but they might be important in analyses of damaging floods at the state or river basin level.
In summary, the NWS flood damage estimates do not represent an accurate accounting of actual costs, nor do they include all of the losses that might be attributable to flooding. Rather, they are rough estimates of direct physical damage to property, crops, and public infrastructure. See FAQ #6 for discussion of the appropriate use of the data sets. Estimates for individual flood events are often quite inaccurate, but when estimates from many events are added together the errors become proportionately smaller.
How can the data be used for research?
When properly used, the reanalyzed NWS damage estimates can be a valuable tool to aid researchers and decision makers in understanding the changing character of damaging floods in the United States. Users of the reanalyzed data are advised to take the following precautions:
We recommend the following procedures to reduce the impact of inaccuracy and omissions in the NWS state damage estimates:
With the precautions noted above, we conclude that the reanalyzed NWS flood damage estimates can be a valuable tool to aid researchers and decision makers in understanding the changing character of damaging floods in the United States. However, the NWS damage estimates are not reliable enough to be a basis for some critical decisions, such as setting precise flood insurance premiums or evaluating the cost-effectiveness of specific hazard mitigation measures.
For more information about the flood damage data, please visit the Data-Related FAQs.