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The present study was undertaken to evaluate environmental effects on average daily gain(ADG), backfat thickness and feed/gain and selection index and thereby to derive correction factors for above traits and index, using 5,048 Yorkshire boars that had been performance-tested from January, 1993, through December, 1999, at the Korea Swine Test Station. The results obtained were as follows; Each test year and month was called according to the last day of the performance test. 1. Effects of test year and test month were significant(P$lt;0.01) in all the traits that had been included in the present study. 2. The ADG and selection index increased each year, whereas backfat thickness decreased. Feed/gain, albeit significantly affected by the test year, did not exhibit any apparent temporal trend. 3. The ADG was greatest between March and June(942.45∼955.89g); backfat thickness was lowest during August, September and November(1.302∼1.314㎝); feed/gain was lowest during March, April and June(2.261∼2.272); and the selection index also was superior during March, April and June(225.07∼226.69) to those during the rest of the year. 4. The selected ratio was estimated to range from 0.782 for April to 0.801 for July when the traits and selection index were adjusted in an additive mode, whereas in a multiplicative adjustment mode, it ranged from 0.733(February) to 0.808(July). 5. By the additive mode analysis, the test month-adjusted ADG ranged from 885.79g(December) to 900.33g(August); backfat thickness from 1.332㎝(April) to 1.339(July and December); feed/gain from 2.320(April) to 2.346(September and October); and selection index from 216.94(October) to 219.09(March). 6. By the multiplicative mode analysis, the adjusted ADG ranged from 886.58g(December) to 903.65g(August); backfat thickness from 1.332㎝(April) to 1.339(July, September and December); feed/gain from 2.320(April) to 2.348(October); and selection index from 217.03(October) to 219.15(March). 7. Rank correlation coefficients between unadjusted and adjusted values were satisfactorily high in all the examined traits and index; they, after multiplicative and additive adjustments, respectively, were 0.903 and 0.900 in ADG, 0.987 and 0.987 in backfat thickness, 0.970 and 0.971 in feed/gain and 0.935 and 0.936 in selection index. 8. The rank correlation coefficient was 1.000 between the selection index which had been calculated using the additive mode-adjusted ADG, backfat thickness and feed/gain and that which had been one-step-adjusted by the additive mode.
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