Normality of egg production in poultry

1. The distribution of survivor egg production has been examined in four control strains of White Leghorn egg laying chickens, a strain of turkeys and a strain of ducks.

2. The most pronounced trend was a persistent negative skewness (an excess of poor producers) associated with a positive kurtosis (an excess of individuals about the mean) and a marked deficiency of high producers.

3. Truncation of these populations at two or one phenotypic standard deviation below the mean tended to reduce the asymmetry but at the expense of the kurtosis which frequently became significantly negative.

4. The greatest departures from normality were observed in the earlier parts of the recording period.

5. A considerable degree of heterogeneity of variance among hatches within strain/years was observed and it is considered that this is largely due to the asymmetry.

6. Negative skewness in the distribution of egg production appears to be a widespread phenomenon and is considered to be due more likely to non‐random environmental factors than fixation of recessive genes.

7. Predictions of progress based on the normal curve must tend to overestimate what is realised and the abnormality observed, which appears to be general, must have negative implications for the accuracy of estimation of biological parameters.