Use of statistics in race determination tests.

The soybean cyst nematode (SCN), Heterodera glycines Ichinohe, has been separated into 16 races (3,5,8). Separation is based on female or cyst (referred to as cyst) formation by SCN populations on four soybean (Glycine max (L.) Merr.) genotypes-'Peking ' , 'P icket t ' , PI 88788, and PI 90763--relative to reproduction on 'Lee' (susceptible) soybean. A soybean differential has a positive reaction (cyst index) to the test SCN population if the number of cysts on it equals or exceeds 10% of the number on Lee; otherwise, the reaction is negative. Often the reaction is given without numerical means and variances. The variation in number of cysts among replications of each differential, including the susceptible cultivar Lee, may be considerable. The index may be near 10 on some differentials. The procedural steps of index calculation are often not given. Mean of a differential may be simply divided by mean of Lee, or each replication of the differential may be divided by mean of Lee. A variance for the index in the latter procedure could be determined, allowing determination of significance of the difference between the index measured and critical index value of 10. The highest number of cysts obtained on Lee among all replications (3) has been used as the divisor instead of the mean number of cysts in some studies on inheritance of SCN resistance in soybean. This has the potential of greatly altering the index value. Variation in various calculations