The ANOVA approach was applied to investigate which factors
significantly affect the response parameters. F ratio is adopted to
examine the significance of the factors in the ANOVA. It is calculated
from the experimental results and compared to the critical F value
(Fcr) which can be found in most of the statistics and experimental
design books [31]. If the calculated F-ratio is bigger than the Fcr
value, the statistical test is significant at the selected confidence
level. In this study,the degrees of freedom for each factor(DOF)
were 2 and total DOF was 8,so the DOF for error term was 0. So, the
variance of error(MSe) that ratio of the sum of squares of error to
DOF of error could not be calculated. Henceforth, it was impossible
to calculate the F ratio defined as the variance of each factor dividing
by MSe. In order to eliminate the zero DOF from the error term,a
pooled ANOVA was applied.The process of ignoring a factoronce