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;Lsing the combination of probabilities method discussed in Appendix 4. the test is based on the assumption that the data has a log-Pearson Type III distribution with a skew coefficient between about ~0.5 and may not be applicable to incomplete records or records with zero flood years. ... C. Refinements to Frequency Curve The accuracy of flood probability estimates based upon statistical analy- sis of flood data deteriorates for probabilities more rare than those direct- ly defined by the period of systematic record. This is partly because of the sampling error of the statistics from the station data and partly because the basic underlying distribution of flood data is not known exactly. Although additional flood data can often be obtained and incorporated in the analysis of flood levels at high flows and rare exceedance levels, pro- cedures for doing so cannot be standardized to the same extent as those used for systematic records. The purpose for which the flood frequency informat- ion is needed will determine the amount of time and effort that can justifi- ably be spent to obtain and analyze historic data, to make comparisons with other watersheds, and make and utilize flood estimates from precipitation. The remainder of the recommendations in this section are guides for use of these additional data to refine the flood frequency analysis. The analyses to include when determining the flood magnitudes with 0.01 exceedance probability vary with length of systematic record as shown by an X in the following tabulation: Analyses to Include Statistical Analysis Historic Data Comparisons With Similar Watersheds Flood Estimates From Precipitation Length of Record Available 10 to 24 25 to 50 50 or more X X X X X X X X X All types of analyses should be incorporated when defining flood magni- tudes for exceedance probabilities of less than 0.01. The following sections describe how to include the various types of flood information in the analysis. 17 f' '.