According to the sample survey results for 200 Shunrei x Shogetsu cocoons produced in 2018, the FFAs (fine fiber assemblies) were non-normally distributed. Interval estimation for the population mean value, based on the observation data, showed that the practical sample size necessary for testing the raw material cocoons for silk reeling was 30. Then, again based on the observation data, we investigated the percentage of cocoons with an FFA value of 20 or more (i.e., LQC [law quality cocoon] ≥20), which was considered to represent a higher risk of the occurrence of lousiness, and found the value to be 29.6%. As the percentage of LQC ≥20 follows a binominal distribution, the result of interval estimation for population proportion showed slightly less estimation accuracy when the sample size was 30. We, however, considered that the mixing ratio of LQC≥20 was an effective index for predicting the occurrence of lousiness hidden in the cocoon filament. The FFA values were obtained by converting the fine fibers separated from the cocoon filament to FFAs and quantitatively counting the FFAs, and it was possible to calculate the predicted number of FFAs per 1 cm² of plainly woven fabric (FFA/cm² fab) using untwisted yarn. The FFA/cm2 fab of the cocoons in this study was estimated to be 188. Thus, in addition to the FFA mean value of the raw material cocoons, the percentage of defective raw material cocoons (in which lousiness was likely to occur) was confirmed, by applying the FFA method to the inspection of cocoons. Once the FFA value was obtained, it was possible to predict the number of FFAs in silk fabrics. It is expected that, in the future, the developed FFA test method will be used as a novel method for determining cocoon quality.