The Reliability of the Soil Burial Method for Evaluating Textile Preservative Treatments

Abstract

The variability in the degradation rate of a fabric treated with each of two rot- inhibiting treatments was measured after burial in a biologically active soil for varying periods of time under controlled conditions. The purpose of the experiment was to determine what limits of reliability might be expected in average strength measurements as a consequence of random variations inherent in the soil burial procedure employed under practicable measures of control. The experiment utilized a random block design, which permitted estimation of the effect of soil variations among the several blocks. The design also made possible an analysis of the results as 5 replicate measurements of 10 observations, or, when combined, as a single measurement of 50 observations. The results showed that the variability in fabric strength measurements increased as deterioration progressed, reaching a maximum when 40-60% of the original strength was lost. The increase was greater in the treated than in untreated fabric, and for one treatment than for the other. The degradation rate of untreated fabric in the several soil blocks showed highly significant differences. But a similar block effect on the treated fabric could not be detected in 8 of the 10 exposure periods. The increased variability and reduced rate of deterioration resulting from the treatment evidently tended to mask the smaller effect of soil variation. At the stage corresponding to the greatest variability, the 95% confidence range for measurements based on 10 observa tions was for one treated fabric 43.8% of the original strength, and for the other, 22.4%. This degree of precision does not justify the high confidence often placed in soil burial test data based on 10 or fewer observations per sample. The variability measurements indicate several measures that can be taken to increase the precision of the test pro cedure by reducing the random variations. The shortcomings inherent in the general practice of reporting the results of soil burial tests as the average percentage of the original strength lost or retained are dis cussed. A novel method of presenting test data in the form of a histogram of the cumulative frequency distribution is proposed as a means of overcoming most of the disadvantages of the conventional methods of reporting data.