Effect of data transformation on the distribution and repeatability of egg-production traits
1995
Ibe, S.N.
SUMMARY: The distribution and measures of departure from normality of four egg production traits, namely 21-30 week (EN1), 31-40 week (EN2), 21-40 week (EN3), and early and mid-period (EN4) egg number were determined using 539, 528, 1067 and 108 records, respectively. All traits exhibited significant (p < 0.001) negative skewness and were leptokurtic. Power transformation of the data with appropriately determined values (t = 3.5, 1.4, 1.95 and 3.6, for EN1, EN2, EN3 and EN4, respectively) generally improved symmetry for all traits including kurtosis for all except EN3. While transformation increased estimates of repeatability of EN1 and EN4 by 71% and 11%, respectively, it resulted in slightly lower estimates for EN2 and EN3, which seemed to show near-normal distribution before transformation. While it does not seem desirable to transform data whose distribution is not far from normal (t < 2), it is desirable to appropriately transform data that show pronounced non-normal distribution before estimating repeatability with them. Repeatability thus appears to be sensitive to data type, and appropriate data (transformed or not) should be used in order to reliably estimate this genetic parameter..
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